As for Wikipedia, it does have an attractive layout, and it's pretty obvious how you can start editing articles. However, I think that anonymity is probably one of the biggest factors to encourage contribution. I hate how everyone on the web wants to know who you are these days. Until I can plug my brain into the internet, I think I will prefer to use it as a tool, not a genuine social space. I've made concessions here and there, but I resent it.