I pitched an Iron Man 2 rewrite where we learn at the end of the film that Mandarin has been secretly behind the scenes and manipulating things since the first film. In my version he is responsible for the stuff with the Senate and Justin Hammer wanting his own Stark Tech because it sets up the Armor Wars. It's been a while since I came up with it but forgot the bulk of the details. Tasha was Shield plant sent by Fury to watch Stark and make sure he doesn't get out of control. Bethany Cabe had an increased role as Stark's budding new love interest.
I agree with Dream's statement regarding the real villain being Tony's drinking problem with I think was handled great in the film. Whiplash and Hammer were surface villains.
Yeah, it was really weird how Mandarin was pretty much ignored in IM2. He was has a very strong background presence in the first movie, since it's heavily implied that he is the leader of the Ten Rings group that kidnapped Tony which forced Tony to create the armor in the first place.
The writers should have continued building on that, maybe even have Mandarin appear at the very end of IM2 to set up the sequel since they seemed to already be planning to use Mandarin in IM3.
At least one good thing has resulted from all this. Iron Man may be the first superhero movie franchise that ends having it's best villain in it's third movie instead of the first or second. Also the Avengers should also have a much smaller presence in IM3, since the Avengers movie would have already been released by then. There would really be no point to advertising them anymore in IM3.