see metafilter, $5 for a new account signup
Thanks - http://www.metafilter.com/newuser.mefi
They give other reasons, but I wonder if it is also retaining the quality of the community. Also, they seem to use public bookmarkin ("favourites"), and not voting as such. Maybe that makes a difference.
problem 2: subscription sites don't work, because they attract too few users
startup idea: a subscription voting site, with smallest possible monthly fee ($1 pcm?) - the purpose isn't to make money, but to limit users. Would probably work best for a vertical market or special interest group.