I've been reading the journals of people who aren't on my friends list quite a bit recently. The reason I mostly post friends-only is not so much for privacy (I generally use filter groups for that) but to keep out the idiots. (Yes, I know about the setting that lets you only let friends comment. I like doing it like this. Also, the fact that you're not on my friends list yet doesn't mean I think you're an idiot. It means you're not on my friends list yet.) This thought led to me writing down some things I do and don't like about comments left in my journal. And now I've written it down, I shall post it. Comments on whether I've managed to communicate my thoughts clearly are welcome. No, comments are solicited. Help me get this into decent shape, and I'll link to it in my userinfo. It's a public post and will stay that way. Edited to add: I think we're covering the "don't post like this" parts adequately; has anyone got anything to say about the "do post this" parts?
If you cause tedium in my journal I will ask you to stop doing that, and if you don't stop after I've asked you lots of times then I'll defriend you. There will be no "final warning" because I find policing discussions tedious in itself. This has never had to happen yet, and I hope it never does. My definition of tedium is the only one that counts for this purpose.
I get very, very bored by short back-and-forth arguments with no time taken to ponder and re-ponder in between commenting. Take your time, think about what the other person said, consider the possibility that you have nothing to add to the discussion beyond the things you've already said. Conversely, give the other person space to think about your point; immediately jumping in with a reply to every new comment they make is a very good way of failing to do this.
I get even more bored by repetition of points that someone made perfectly well the first time (I have no problem with thinking out loud, and I positively love it when someone says something like "hmm, I said X earlier but now I'm starting to think Y is more accurate", or "I've changed my mind"). I like people who understand the difference between clarifying a point and trying to make someone else accept it by reiterating it twenty times.
Single comments are never tedious, even if they're relatively content-free. Obviously-non-hostile comments are rarely tedious (the definition of "obvious" is "obvious to Kake"). Social grooming comments (that is, content-free comments used to reinforce social relationships) are not tedious unless excessive. I have no objection to *hugs* comments, but if I've not asked for comfort or hugs then I probably don't need them.
I hate the way that livejournal switches to a horrible UI when a post gets over 50 comments. If a discussion is approaching this limit, I'd prefer that social grooming comments and other less-relevant comments are taken elsewhere ("elsewhere" includes other posts on my journal), or at least postponed. Once we've hit the limit we're screwed anyway, so anything goes after that.
I love it when I get a comment on a six-month-old post. I love it so much that if it's substantive I'm quite likely to make a new, brief post pointing people at the old one, so your efforts are not wasted. I like comments that say "yes, that makes sense" and nothing more; they let me know that you've read what I had to say.
I like comments that disagree with me intelligently. My definition of "intelligently" is the one that counts; it includes things like being coherent, and showing evidence of having put a lot of thought into the matter. I also like comments that acknowledge ignorance and request elucidation.