Perhaps the simplest way to convey emotion is to describe in brief the emotional reaction in question. A link to a news article, presented without comment, gives your readers no clue what you're thinking about it (unless they already know you). Saying "This makes me happy!" or "Permit me to rant and rave here, because this makes me angry beyond belief", gives emotional context for what follows.
Another common method is through the use of emoticons, small bits of text or tiny images that convey a simplified facial expression or situation. This can be as simple as a sideways smiling-face -- :) -- or as complex as a tiny animation depicting one smiley-face knocking another over the head with a club and dragging it out of the frame, presumably to a cave. Textual emoticons are more prevalent on LiveJournal than image emoticons. LiveJournal does include a small selection of emoticons that can be used when leaving comments and will show up in the comment's information bar (click on the small grey smiley-face next to the Subject: field in the ?mode=reply view when using site-scheme comment pages, or use ?mode=reply&format=light to override custom comment pages), but other emoticons require a location to host the emoticons and basic HTML to include the images in comments or entries.
LiveJournal offers the ability to set a mood (text and image) for each journal entry. There are a number of built-in mood icon themes, and Plus, Paid and Permanent accounts may upload their own mood themes. A mood icon theme allows the ability to assign a picture to a pre-set mood, and then select from the pre-set moods when creating the entry to choose the image, and then keep the pre-set mood text, or overwrite the pre-set text with some custom text. The mood for this entry, for example, is the custom mood icon I have assigned to the pre-set mood "creative", but I have entered custom text declaring my current mood "informative". Only one mood may be chosen for each entry. Music and Location may also be set for each journal entry. While this may be intended to be interpreted literally, these fields can also be used to give further context (I have chosen as my current music the actual ambient sound, but have taken a more whimsical approach to my location). However, mood icons (and music and location) are not available when leaving comments on LiveJournal.
LiveJournal also offers the ability to upload multiple user picture icons (userpics) for each account, limited by account type. Userpics can range from simple avatars intended to represent the user, to a variety of avatars showing emotion, to non-avatar userpics conveying an emotional message, or images without a concrete meaning that are nonetheless symbolic, and off into the land of the completely inexplicable to outsiders. I have chosen a userpic that embodies a sardonic introduction to the internet. The fact that it also makes reference to a previous piece of writing is less obvious, but still significant to me. Sometimes a single image is used with multiple meanings. Sometimes people assign different keywords to the same image to better label the separate meanings. Do be aware, when choosing keywords for a userpic, that others can see the keywords you are using (depending on their style).
Body language is one of the missing elements in a text-only forum. Some people get around this by describing the actions that they are taking, or would take if everyone were in the same place. Some actions are well-known and stylized, such as the acronym "LOL", Laughing Out Loud. A short action placed between asterisks often denotes that this action is being taken, for example -- *hugs* would indicate that I am (or would) hug the person to whom I am replying; *smacks over head with pillow* is another obvious action. Some online forums encourage people to get elaborate with their poses -- "Miss Lunatic perches on the edge of her chair, leaning intently into the keyboard, a frown of concentration wrinkling her ivory brow." This level of attention to the physical is rarely seen on LiveJournal, and may mark one as an outsider. And a caution for any online forum -- take care when your actions describe an effect to another person. Even though *stabs you with knife* has no physical effect to the other party, it may still come across as an act of violence unless the forum takes physical threats as all in good fun.
The other missing element in a text-only forum is tone of voice. While it's hard to get subtle distinctions in tone of voice like the difference between seething and irate, it is possible to emphasize words and phrases. ALL-CAPS IS CONSIDERED YELLING. UNLESS YOU WANT TO BE THE INTERNET EQUIVALENT OF THE CRAZY GUY AT THE BUS STOP WHO SCREAMS EVERYTHING, CAPS LOCK IS ONLY FOR TELEGRAMS. You can emphasize Very Crucial Words with initial capitals, however, and write some (but ONLY some) words in all caps. You can also use HTML to <u>underline</u>, <b>bold</b>, <i>italicize</i>,
With the basic tools of careful word choice, stating your emotion, emoticons, mood icons, userpics, body language, and text emphasis, you should be ready to interact and make sure that your words are coming across right.
When all else fails, a picture of a cat with an appropriate caption usually gets the message across loud and clear.