cos

I've been using Connotea, a free open source social bookmarking service hosted by Nature Magazine, for years. I'm just an IT worker, not their target audience of researchers, clinicians, and scientists, but it works just as well for me. It's had the tags intersection and RSS-ability you mention, the whole time I've been there.
E.g., http://www.connotea.org/tag/research+education
http://www.connotea.org/user/shivginthernkr/tag/boston

Easy instructions for importing Delicious to Connotea:
http://blogs.nature.com/connotea/2010/12/17/how-to-import-your-delicious-bookmarks-to-connotea