Originally we were looking for a free wiki that had OpenId signup so that there was quite a minimal barrier to participating in the wiki. The OpenId feature was later canned and the ads were quite intrusive at the same time as the ad-free subscription cost being quite expensive.

I have had a look at using other wiki farms but for the people who have signed up there is nothing along the lines of the comments and the multiple author levels on Wetpaint. To a certain extent there is just a certain amount of inertia now. Until something compelling comes along we will probably continue to use Wetpaint.