Barb Dybwad's list of suggestions for social bookmark services is good stuff. I can't really do it justice with a summary so just go read it. (And I really hate posts that say that.)
Her list points out one of the issues with centralized Web services. Since these aren't pluggable applications, the rate of innovation is gated by the developers who have access to the backend code. Web service APIs are a start to building new applications but from what I've seen, apps built on top of them rarely get folded back into the main service.