I go through phases. I've been posting a decent amount recently (I think), but a month ago I barely even visited this site.
For me, the sheer number of threads to sift through is overwhelming, and I end up losing track of what I've commented on and what I haven't. It's difficult to keep up a dialogue like that (there are one or two that I remember well enough to keep tabs on). That's probably the biggest reason. Once I've found an interesting thread, I then have to read through all the other responses, and by the time I get to the end of some of the longer ones, it's been pretty well talked-out (like some others have already said, I'm more likely to contribute when less people already have). So, to sum up: lots of sub-forums - lots of threads - lots of comments - so much to take in I don't feel like adding to it.
I'm more a reader than a writer at heart, anyway; I learn best by reading and internalizing; I joined this site to learn.
Also, I see that a lot of people feel judged, and that's sad.