I agree with you regarding the stuff with River Song. When she appeared in that lone season four episode, I thought introducing a future companion of his who knows him well but who he has never met was kind of a cool idea.

But her first appearance in season five made me do a 180 on the whole thing. She was suddenly a really irritating character, very obnoxious, and hairy_hen is quite right in that there is absolutely no chemistry on screen between the two. She is very demanding and condescending (she actually kind of takes away a lot of the appeal of the Doctor to see him reduced like that) it is really hard to imagine the Doctor ever falling in love with her.