Well, I'm not going to speak for him but here are some reasons.
1 It's a mistake making Erik's adversary a mutant. The whole point of his arc is that he against human movements that judge mutants, which is why his first few scenes in the Nazi camps as a victim of that experiment worked. But he is not anti-mutant so why make his adversary a mutant, let alone an uber-powerful mutant.
2. You don't need to connect the dots and tell us how Charles was paralyzed in this film. It was tacked on for the benefit of a "stupid audience's benefit.'
3. Mystique was better as a mysterious assassin type in the original films. Showing her as a child looked silly and took the mystery out of the character.
4. The girl that flies (forgot her name) looked damn silly.
5 The film's structure was wrong. The training montage came in the LAST REEL of the film. I think that the Cuban Missile Crisis and the Mutants revealing themselves to stop a war should have been the climax of act one or act two, not the climax of the film.
thats for starters.