1. Bryan Singer directed the other two, only to be replace by the hack who brought us "Rush Hour" 1&2. Enough said.
(Spoilers)
2. They kill two crucial characters. Two indispensible characters, and treat one of them as a horror film victem, rather than a character from the first two films.
3. They continue to butcher Rogue's character.
4. They underplay the Pheonix subplot to the point where the whole thing is utterly laughable.
5. Magneto seems to just be up to his old tricks.
6. Lameass plot.
7. They butchered the character of Juggernaut. If you're going to include the Juggernaut, do it right. Professor X and the Juggernaut have a past that goes as deep and Prof. X's and Magneto's.
8. The only character they got right was Beast.
9. They picked Halle Berry, the single worst actress in the entire film, to be one of the people who carries the movie.
10. They undermind Cyclos' character...again.
11. Too much Wolverine. Not enough of anyone else.
12. Who the hell is supposed to be the protagonist. The other two films had a focus. This one was off in some random direction.
13. Little substance, little plot.
14. Hell, the little substnce and plot aren't even replaced by much action, which would actually have helped.
15. Not enough Angel.
16. No Nightcrawler. Enough said.
17. Not enough Mysterique.
18. Not enough Prof. X.
19. Not enough Pyro and Iceman.
20. Disappointing Pyro vs. Iceman showdown.
21. Disappointing climax.
22. Too much Wolverine and Storm (I cannot stress that point enough).
23. Complete butcherment of Storm's charcter.
24. Wolverine too soft.
25. Not enough humor.
26. Too much unneeded angst.
27. The action weren't even well-done.
28. Magneto's relationship with Professor X. non-existant.
29. Magneto not showing enough remore over Professor X.'s demise. In the comics, he'd have had doing an all out lamentation.
30. The Pheonix would never have worked with Magneto.
31. Scott should have been the one to save Jean.
32. Hell, Scott should have taken center stage. They have yet to even give Scott even one limelight moment. Too much Wolverine (never understand the appeal in that jackass).
Like I said, some time in a gas chamber would do the director some good.