Open source usability
Newsforge has an article on one of the biggest weaknesses in the open source model - user interface design. The article actually encapsulates some of the problem that the open source community has in this respect. UI is not purely a technical problem in that you can't solve it with abstract knowledge. It is hard work to create a really usable interface and involves a lot more face-to-face human interaction than the open source community can support. I don't think it is possible to build a really usable interface without some kind of in-person user testing. It doesn't have to be very elaborate or costly, but it has to happen.