Personally, here are two things that prevent me from participating as much as I’d like that I suspect might apply to others:
1) Internet addiction
Whenever I post or comment, I can’t help but start checking in to see if I get any responses rather obsessively. But I don’t want to be in that state, so sometimes I withhold from posting or commenting. And more generally, being on LW means being on the internet, and for me, being on the internet tempts me to procrastinate and do things I don’t actually want to be doing.
2. Relatively high bar for participation
If I’m going to comment or post, I want to say something useful. Which often means spending time and effort. a) I have a limited capacity for this, and b) if I’m going to be spending time and effort, I find that I often see it as more useful to apply it elsewhere, like reading a textbook.
With (b), not always though. There’s a lot of other times where I do feel that applying my time/effort here on LW is the most useful place to apply it.