Hmm... As has been said, I don't think there is a site like that...
However...
If you go to amazon.com (not .co.uk) and type in a movie, you'll see the results reveal 'tags' that people have added that give you a general idea of what to expect from a movie; for example, 'Bridget Jones' come out with 'romantic comedy' and 'Movie Night'. It's not great yet, but it might provide some interesting information 
