I think I'm getting the hang of this

It's been about five weeks that I've been here, in this virtual space called LiveJournal. I've figured out how to post, to cut, to join communities, to make my own styles, etc. Technical stuff -- no problem. But I still have some questions about the conventions of posting.

1. Replying to comments
I love the comments function because that's what makes everything a conversation instead of just an essay. But I've never quite worked out if I should reply to every comment or just when I've something brilliant to say in return.

What do you do? What do most people do? What do you think I should do?

2. Posting fic
When posting fic in LJ, as opposed to posting a link, what guidelines do you follow? How long does a story have to be before you decide to link instead of posting? (And maybe there's a hard limit -- I've not checked.)

What about rating? Do you post all fic up to NC17 or do you draw the line somewhere? Does it bother you when other people post/don't post NC17 stories?

3. What else have I been doing wrong?