This article is a really good summary of some of the things that have been lost in Wikipedia over the years ranging from the ability to solve disputes simply to the ability to do text dumps into an article trusting that someone else will come along and wikify it.
On that note, for anybody interested in a good history of the site, I’d recommend “The Wikipedia Revolution: How A Bunch of Nobodies Created the World’s Greatest Encyclopedia“.
I was thinking about this the other day and I think that if I had to choose, the only sites I truly couldn’t live without would be Wikipedia and YouTube. I’d hate to lose Google but there are other search engines out there. Facebook is handy as sort of a modern address book/party line but again, I think I could survive without it. I look at Twitter more than I ever did before but it’s still peripheral to much of my Internet use. Reddit and MetaFilter would probably be the hardest sites to leave behind but there are lots of other aggregators too (although Digg is having one of their annual revolts rights now).