What particular factors might have begun to inhibit participation in 2006, when we know it began to stagnate?
The data presented from the earlier fact base work indicates that there doesn't seem to be a relationship between new article growth and participation in En, FR, De wikipedia:
What this data says is that participation tends to plateau after a period of time, but that the most active contributors get even more active in expanding content. I will also look for a visualization that Eric did - think it shows a similar take-off.
The data on increased reverts from Ed Chi speaks to the original question that Netmouse posed. This data actually raises a question about whether there really was much of a shift in 2006? Looks to me like there has been a rather consistent increase in reverts over time. This may speak to continuous tightening of editorial standards/control across the project over time rather than a one time increase.
One other helpful source of article growth info is Eric Z's visualization visualization