I think the reason is that internet is full of trolls and if you don't put significant effort into making community / collaboration clean and vibrant then the collaboration site is destined to fail.
For example, compare Quora and Yahoo! Answers. I don't thing Quora is successful because they have very good UI (it does not hurt), I think they are successful becasue they are very serious about making they site clean and without trolls. On other hand, Yahoo! Answers is 'troll heaven'.
For example, compare Quora and Yahoo! Answers. I don't thing Quora is successful because they have very good UI (it does not hurt), I think they are successful becasue they are very serious about making they site clean and without trolls. On other hand, Yahoo! Answers is 'troll heaven'.