What particular factors might have begun to inhibit participation in 2006, when we know it began to stagnate?
Yeah, I've always thought that the low hanging fruit explanation is, at best, a partial explanation. There are definitely other factors at play, some external, some internal. ... and we can really only have an impact on the internal factors.
As far as I know, some Wikipedias are more liberal, some are more tight, and this has had very little impact on article or editor growth. The real issue is cultural and behavioral, and a culture acts without policy (sometimes in spite of policy).
I think there is much more support for the hypothesis that there is more friction in the community, due to its increasing size, and maybe due to the kinds of people it attracts, and the kinds of conflicts that have emerged due to Wikipedia's popularity. Check the rise in administrator incidents, and then use the same tool to look up Wikipedia:Wikiquette alerts. People are just fighting a lot more, and I'm not so sure that we've been able to address the real root causes of those fights.
I'd like to take a closer look at Erik's stats. It's unfortunate that the best study I've been able to look at has come from an external source. I think there are some important trends worth looking at.