Metafilter has a one-time, $5 signup fee, plus restrictions on new users to post content:

http://www.metafilter.com/newuser.mefi

Unfortunately, by raising the bar to participation, a lot of people who would be an asset to the community are prematurely excluded.

My suggestion for dealing with trolls and flames is applying a "three strikes" rule: If a new user's average comment rating drops below negative two, their account gets frozen for a set period of time. Or, you could apply some sort of "three strikes" rule (three comments -4 or below result in a freezing).