Usability Yes. Social Features, Uh-Oh.
Fragment of a discussion from Talk:Question of the week/Archives/2009-12-14
The English Wiki have Miscellaneous For Deletion (MfD) that plenty enough to dissolve "problematic" groups.
Fears come from how much damages such groups will do before being stopped. There will be some groups which will require local Arbitration Committee sanctions :(
That said there will also some positive gains, we are all just crossing fingers that "issues" will be kept in check.