cheerfulSocial meme I think is generally helpful: on the sort of post where you wind up not knowing what to say, but know that you should acknowledge that you are present and have been listening, commenting to acknowledge that you have read the post. Some people (who will generally tell you so!) react very poorly to 'nethugs.
An idea for the burning question of "How should sponsored communities ethically promote themselves on LJ?" could be a sponsored_promo community, in the style of
community_promo. They're clearly not regular communities, quite, and it would not be fitting to clutter up an area that was built for regular communities only. (Or, hell, direct them to shameless_plugs!) 
trystan_laryssa came over last night, and we had a giggly good time. More on that later. Then Snarky Lady misdialed at about 6 in the morning. Another volunteer talks about why she volunteers for LJ.
I need to do some poking at my style, because I have found that when I screen a comment, there is no link/text to unscreen it. I fear it's the same with freezing. I of course could flail about and get the unscreen URL and paste in the correct comment ID, but that's cracktastic and I don't want to do it.
