"Usability" doesn't mean that everything should be easy. Many of the things to consider when building communities is how to discourage things you don't necessarily want.

For example, allowing anonymous users makes it easy for people to contribute without registering. If you want as many comments as possible, you'll probably allow anonymous comments. Of course, that means that there's little accountability. If you'd rather not allow people to drive by and flame or spam, maybe you'll require people to register.

Besides that, there's a school of thought that says, "Why should I build a system that doesn't really solve a pressing need if no one's going to use it in six months?" One example came up in my book. You could keep track of user features and task details in a nice, web-enabled system with access controls and workflow. Maybe that's appropriate. A lot of teams do really, really well just by using pens and index cards.