DoryDarko
There can be many reasons why the original director of a franchise may decide not to return for a sequel, though they all usually have in common that they're very Hollywood-politically-correct. So exactly why Jon Favreau decided not to helm the third instalment of what is, for all intents and purposes, 'his baby' Iron Man, I can't be sure. But I can tell you one thing: something definitely changed.
Everything about IM3 feels different from its predecessors. The atmosphere is darker, which is not to say it's really just a lot less humorous (which is let's be honest precisely what we love about Iron Man). The new characters are terribly underdeveloped and overall it's just a great deal less interesting. The storyline was completely vague...