I'm with ya - I've replied to a few of those threads, and I try to keep my criticisms constructive. I would hate to see something like "The AEPhi's at my school are a bunch of skanky ho's!" so I wouldn't say that sort of thing about the other sororities represented at my school even if it were the case. Besides, by saying this sort of thing you come across as catty - and that reflects badly on you and on *your* GLO (if applicable).
Just my $0.02
Edited to add: Apparently even my slight criticisms have rubbed a few people the wrong way...