He doesn't hate it. RDJ said there were certain aspects that were dissatisfying and disappointing to him about IM2. And I think it's a fair assessment imo.
Downey also admits that, despite the box office success of Iron Man 2, there are elements of the final film that he's not entirely satisfied with. The aim for the third go-round is to "leave it all on the field" and fall back on the natural rapport between Black and himself.