I kind of like the postiive/negative rating. It gives a general overview of community opinion which then gets fleshed out by the discussion that follows (which is what the forums are all about!).
I also like how entropic the positive/negative ratings are -- the urge to be contrary that it pulls from individual members.
The polls on the inside of the story might cut down on spam, I suppose.
More likely, though, they'll just give members an opinion they think they believe instead of forcing members to think out and revise their own opinion through writing, editing and posting.
I love MR because so many are so thoughtful. By suggesting "answers" or "opinions," I'm concerned that polls will reduce that thoughtfulness.