What particular factors might have begun to inhibit participation in 2006, when we know it began to stagnate?

These are hard questions. When I first started editing there wasn't really any citation method in place. I just used to gambol around putting in what I "knew" and I wouldn't run into any resistance. My edits would stay even though I provided no citations. Things became more strict but I sort of grew up with things becoming more stringent in such a way that I just adapted as things became more "difficult". So I'm trying to think how I would have responded if, when I made my first few edits, I had been reverted with the summary "unreferenced". It's hard for me to see through the eyes of a newbie. That's something I'm going to have to try to do to come up with good answers for this task force.