I think one site I'd like to be able to import from is
FantasticFiction, selecting an author and importing there whole catalogue or sub-section (series/novels) would be handy.
This would have been even more welcome a couple of years ago when just started using calibre, now my collection is nearly sorted it's not as useful (to me) - unless I come across a great new author that I haven't read before and they have quite an extensive collection.