I think that not allowing spaces causes tags to be come diluted. Instead of the tag "social bookmarking" you end up with either socialbookmarking, social.bookmarking, social_bookmarking, social-bookmarking. Something along those lines. The same tag has now split out into a large number of posibilities. As far as I know, only Delicious (among the big name bookmarking services) still does not accept spaces in tags.
Otherwise the answers sound good.