Money?

Metafilter seems particularly relevant in light of Quora and App.net, and while the price tag has kept it obscure I've only heard good things about its community. So I'm curious what criticisms you have.

You lose access to answers from the sort of people who won't spend 5 bucks to get onto the site.

In some circumstances, that may be extremely relevant.

Or anyone who doesn't want the admins to have their True Name via paypal.