Most sites do recommendations as a big Venn diagram. If my_upvoted_items intersection your_upvoted_items > x then recommend me your_upvoted_items union my_upvoted_items(complement).

I don't think this is a great approach. If you and I have similar tastes, chances are that I have intentionally avoided the things that you like but that I have not voted on.

I'm working on a music recommendation site that, I hope, will avoid this problem.