I don't use social bookmarking, it has never appealed to me. Browser based bookmarking has always been sufficient for me, and I don't spend much time "browsing" the internet, I am typically trying to find something specific, so a general search engine is usually sufficient for me.

Some features that might make me more likely to use social bookmarking: OpenID accounts, really simple and good search, RSS for tags (no doubt this already exists), syncing with my bookmarks on all the browsers I use and aggregating them in one spot, letting me save my bookmarks to a bookmark file readable by Firefox... thats all of the top of my head, although fundamentally I don't think they fit in my life flow :/