Signs you might not be from LJ originally (some of LJ's rules, both social and not-so-social)
People in LJ tend to cluster into the same sorts of social groups that people face-to-face do, with the same kind of evolved social standards. Some of these are going to be well-nigh universal. Some of them may vary widely depending on the social group you've fallen in with, or have stumbled into. There's the potential for just as much backstabbing and drama online as there can be face-to-face, just different kinds.
Check the profiles of your friends to see who their friends are before talking smack about someone you know and can't stand. They may be in close with them. Then again, they may just be reading the insufferable bastard (no gender implied here) for obscure reasons of entertainment or out of social obligation, but talking smack about someone on the friends-list of your friend without feeling out their comfort level with that is just about designed to start something.
That "friend" thing. If I list you as a friend, it means either a) I like to read your writing, b) I trust you to read my locked-down stuff (at least some of it), or c) both.
It doesn't mean that I think that you think of me as a friend. There are people who I have listed as friends who may not have ever noticed my presence, or who may not remember me well and think of me as a cordial distant acquaintance.
Or we may actually be friends. Who knows.
When you add someone as a friend, it's generally polite to inform them, especially if they have a small number of friend-ofs, and doubly so if all their friends are mutual. Some people don't care. Some people do. Use your best judgment, but a "Hi, your writing looks nifty, I've decided to read it; I found you via ___" is rarely considered out of place if public comments are enabled.
[Edit: that friend thing. "Hi! I saw you and you're nifty! I'm adding you!" is absolutely not the same as "Hi! I saw you and you're nifty! Can I add you?" The former is an optional courtesy. The latter is a big red stamp across the forehead that says either NOOB, or DUMB-ASS NOOB WHO CANNOT READ, depending on whether the person being asked has a friending policy in their profile that says that anyone may add without asking. More discussion in comments. ]
[Edit: Friend rules. Different social groups have different friending/defriending standards, and if you assume that the standards that hold true in your group are obviously going to apply to their group, you're in for a world of social awkwardness. A stated friending/defriending policy from another user, usually as written or linked from their profile, trumps all other points of etiquette that you may have learned elsewhere. Their journal, their rules. ]
That "friend" thing. If I remove you as a friend, it may mean that I just don't need to see you on my friends page for whatever reason. It might be that you write ten novels a day, and I can only handle reading one. It might be that you have these huge images that break the rest of my friends page. It might be that you've just done the thing that's put the last straw on and snapped my patience clean in two. It might be that I don't feel particularly close to you, and am just not reading. It might be that I'll go over to your journal from time to time anyway. It might be that I don't really have any locked posts, so there's nothing to miss. It might mean that our friendship really is over. But you never know.
It is considered polite to let a person you're removing as a friend know why you're doing so, if you're doing so for a reasonably socially acceptable reason like OMG YOUR PAGE-BREAKING IMAGES. No hard feelings, eh? It helps cut down on drama. It's a relief to know that someone's no longer reading you because of your penchant for really, really long uncut entries, or the fact that they added you for fandom reasons and they're really no longer following that fandom, vs. the possibility that you said something that pissed them off, and they're just not telling you and they hate you forever.
On the other hand, if you hardly know someone, and you're reading them because their entries entertain you, and all of a sudden you realize that you're no longer comfortable reading them because in fact they are creeping you the fuck out, there's absolutely no need to let them know about this. It'll make them uncomfortable (okay, maybe you don't care about that) and likely make you look like an ass (which you might care about). Unless they're the kind of public figure who legitimately needs that kind of feedback when they're alienating their audience, in which case an email is still probably going to serve you better than a public comment.
Non-mutual friending! Some people actually care about making their friends match up with their friend-ofs. The existence of non-mutual friends drives them up the wall. I have no insight into this, and I don't think I want any.
Serial adding, and other forms of unrequited love! Some people think it reflects badly on them to have someone who is the blatant antithesis of everything they stand for listed as a friend-of on their profile. No LJ citizen is necessarily going to think poorly of you for having picked up the attention of someone vile at one point in time, but it helps that you can conceal them from your profile by banning them. If someone's not an LJ citizen, they may indeed think poorly of someone for that, but that's the view of an outsider, in which case it only really counts if they're your mom, or law enforcement.
There are some people who like to create assortedly distasteful journals and add people as friends to see who gets wound up. These are serial adders. Like spammers, if you tell one to go get ridden by the horse it rode in on, you'll probably get ten more. This is very much a tired social practice these days, and not much followed, but it was big for a while.
On the other hand, if you've picked up the attention of a disproportionate number of detracting followers (disproportionate, not necessarily large -- it's all relative) that can be an alert that you're a drama locus and have attracted the negative attention of the wrong kind of people for the wrong reasons. There's no telling.
Someone's LJ is a little bit like their living room, or at least their garden party. Some places don't do at all well with gatecrashers. Some places, the more the merrier. Unless it's a large party, it's considered good form to introduce yourself briefly to the host(s), with some indication of how you came to be there -- your life's story is not necessary (and probably not really wanted, either), but mentioning "I saw a link ____ and hopped on over" is rarely out of line in your first contribution to the discussion.
Someone's LJ is a little bit like their living room. You don't tell your friend's other friend that they are five different kinds of unprintable thing right in your friend's living room. You take it outside, unless the other friend is being such a blatant ass that there's really no other choice.
If someone has disabled comments on a journal entry, chances are they don't want to have to field comments from the general public or the viewing audience, if the viewing audience is smaller than the general public. Unless you know them well enough to be reasonably assured that they won't take it ill if you contact them through other channels, don't. (If you do know them well enough to feel it's appropriate, or if you know that they have other standards, act accordingly.)
In a flat message-board environment, comments are presented in strict chronological order, and people keep checking back when new comments are added. If the person you are attempting to reply to is monitoring the discussion at all, they'll probably get notified at each new comment, and catch any replies to their contribution.
In that kind of environment, reply-to-specific-comment-in-thread and reply-to-whole-thread distinction is mostly for cosmetic effect and quoting. LJ is different. If you want any given person to know you've replied, reply to their comment directly. This is a very important part of community interaction on LiveJournal, and there are very few quicker ways to make yourself look silly, out of place, and socially incompetent. It's the internet equivalent of attempting to carry on a conversation with someone but making no eye contact and yelling at the wall instead of actually talking to the person you're trying to converse with.
Signatures. LJ doesn't deal with signatures much. People sign their emails. People sign their message board posts. People don't tend to sign their LJ posts and comments, not unless they come from somewhere that did teach people to sign their online interactions.
Consider what you're going to say before you post to a community with people you don't know. Take a look over the community profile and the other posts before jumping in. Unless the community is very very kindly disposed, everyone will tell you if you've broken the rules. Fifty times over.
It's one thing to break unspoken rules. It's entirely another one to break rules that are explicitly stated on the community profile.
Commenting with unrelated material to a post, either in a personal journal or in a community, is generally some form of misstep.
It's considered rude if it's done for the purposes of thread hijacking -- say if you wanted to ask the denizens of note_to_cat if anyone had any experience with a particular flea product, you would absolutely not go into the most recent post and put a comment in there. Instead, you would create a new (top-level) entry in the community and ask there. (Community rules would dictate that the bulk of the entry be behind a cut if it were not formatted as a note to a cat, in that particular community. Abide by your local community's rules.)
It's considered a little intrusive if it's a personal journal, especially if you don't know the person. One softens this by stating that you know that this is unrelated material before launching in. If it's sufficiently nifty material unrelated to the post, all awkwardness may be lost. If you're actual friends with the person, rather than just LJ friends, you can comment just to say hi without awkwardness, but that's typically a liberty reserved for friends.
Unrelated material, the extremes! If you come in to a post with really unrelated material, and you're a stranger, you'll probably be treated as a spammer. If you come in to a post with related commercial material, you'll probably also be treated as a spammer. LiveJournal is not a commerce-friendly site.
Intrusive text formatting is frowned on.
Do not resize your text to a fixed size without very good reason. (Relative size is a more flexible thing; small-formatted text is accepted in some circles as "whispering"; struck-through text is accepted in some circles to indicate "I didn't say that, really I didn't!") Don't change your font. Don't change your color. Any formatting that takes up the whole post or whole comment is (generously) considered a coding mistake, (indulgently) considered vanity or the action of a newbie, or (typically) considered rude.
Why is it rude? People view LJ in an enormous number of ways. Unless you are writing for you and yourself only, on your own little soapbox, without considering the needs of your readers, you're not being socially minded. Yes, it may just be that you're making sure that your text shows up as pitch-black wherever it's at.
Congratulations. You've just rendered your text unreadable to the person with the black background. Not only that, but you went out of your way to do it. Yes, they may be able to read it with a little work, but the fact remains that you made it harder for them to read, and it was a change you made deliberately, and they won't thank you for it.
Some people may not be affected or only minimally affected; some people would only have to squint a little; some people would have to go out of their way to make it readable; some people, especially visually impaired people and blind people with screen readers, may be completely unable to read whatever it was you wrote.
Any imagined cool-factor your precisely-chosen size/font/color combination is intended to create will be overshadowed by the fact that you're violating the social standard. Something like this can be overlooked in your own journal, because it's your journal, and no one else has to read it. However, since you can modify your journal's style to display how you prefer it in your own journal without affecting anyone else, having to resort to markup tied to the text in order to get the effect you want is seen as anywhere from silly or eccentric to rockheaded stupid. Posting to a community with altered text, or posting comments with altered text, is a profoundly antisocial activity. There may be isolated pockets where altering text is accepted or even encouraged, but it's a standard that even known trolls rarely violate.
Excessively long, wide, markup-intensive, and/or bandwidth-intensive entries get <lj-cut> under most circumstances. So do items that are of dubious safety. LJ has a lot of standards about being responsible to the community as a whole.
Userpics. Unlike a message board, where a lot of things are done in emoticons and signatures, LJ expresses emotion and flavor-of-the-day through userpics. They are not just an avatar of the journal owner. They are a flag. They indicate mood. They contain useful meta-information about the discussion. If, say, I'd used my "flaming asshole" icon to put on this post, I probably would have been writing this as a rant in passive-aggressive response to something that someone did, but I didn't feel that I could tell them directly that they're pissing me off. Instead, I'm using my "LJ fudge" icon, which indicates LiveJournal, community spirit, friendship, and good times. Not to make anyone paranoid about me when I'm using that icon! ;)
Respect the lock. What happens behind locked entries stays there. There are various sets of rules for different circumstances -- locked communities with open membership are fair game, journals with a habitual friendslock for general not-being-all-over-the-internet reasons are like any other private person's personal life (gossip at your own risk, don't spread the wrong things the wrong places), journals locked to defeat stalkers or for other reasons of privacy should stay that way, deliberately locked entries in otherwise public journals are locked for a reason, and telling Bit that Anna has just made a filtered post trashing on her is downright cruel and can lead to you being banned so hard.
Don't link to locked posts with identifiable information about the contents. Say exampleusername had a locked entry about depression, and you made a comment you thought was really insightful and worthwhile, and you wanted to repost it in your own journal. It would not be appropriate to link to that locked post when reposting your comment, not without permission, assuming the comment had anything to do with the topic of the entry, and sometimes even if not.
If in doubt, don't spread it around. You don't want a reputation for not respecting locks and filters. Really.
Journals are for posting in, if you live here. If you don't post in your livejournal, like, ever, you're treated as if you don't belong here. (barakb25, I'm looking at you.) This is because you mostly don't belong here. You don't know the culture, you don't know the people, and you're not driven to chronicle the same way the rest of us are.
Even if you only do have the journal for the purpose of commenting, or of reading the locked entries of your friends, it is polite to post to your journal at least once to announce this. Comments may be set in any which way, but there should be at least one public post. Even completely private journals should be posted in. It really unnerves LJ citizens to see a journal that has never been posted in. The casual user may never notice, but we'll know.
[Edit: replies! When replying to someone's comment to you, always hit the "reply" link to that comment, and never the main "reply" link for the whole post. Sometimes weird issues will cause you to accidentally reply as a top-level comment, and that's regrettable, but not your fault. "Replying" to someone else but not using the reply link on their comment means they are never notified that you have replied, which is an integral part of LJ social interaction. People depend on these notifications to continue discussion, and may not ever revisit the post without that notification. Plus, it breaks threading. There are legit reasons to reply to the main post and address issues brought up in comments, but if that is intended to be a reply to any of the commenters, at least drop them a reply letting them know to see the full reply at top-level.]
[Edit: If you aren't reading someone regularly, and they don't know about this (and you don't really want them to know), and they say something that baffles you, go get caught up on their recent entries (if the context allows it) before you ask what's up. Otherwise you risk blowing your cover about not reading them.]