One of my favourite things about using "user generated content" or "web 2.0" tools for education is that it allows for asynchronous interactions with (and between) students. They can ask questions or even just interact with the course content any time they have an Internet connection. Not that these sorts of tasks can't be accomplished using email and basic web pages, but allowing the students more power and responsibility for things like editing a wiki can perhaps encourage motivation.
A related issue, though, is how many of these tools are blocked by school networks. My district has been very good about allowing YouTube, Wikipedia, Twitter, and others, but I imagine that is not the case everywhere. It would be interesting to have a study done on the educational impact of allowing different types of sites in a school situation, similar to a study that found employess who use the Internet for personal reasons are 9% more productive.