I had no idea people hated "X-Men: The Last Stand" this much. I didn't care for it either, but I'm surprised people are writing vicious little essays tearing it apart, much like I do (because I can't resist) any time the subject of "Spider-Man 3" is brought up. I didn't think it had as many stupid elements as that one. It just paled in comparison to its predecessor and tried to cram two plots into one film, when one of them (handled more carefully) could have easily carried a great film. My only major objection to the movie was its killing off major characters in a cheap attempt to generate drama. The rest of its missteps were minor, as far as I'm concerned.
One good thing to come out of it was that it inspired Ratner to say the one thing he's ever said that I actually found clever. Upon signing on to direct the movie, he remarked, "I am not Joel Schumacher". I thought that was pretty funny. I believed it might be a good sign that he was aware of what mistakes have been made when the reigns of a successful franchise were handed over to another director in the past, and would therefore be able to avoid repeating them. That wasn't quite the case, but I thought the general consensus was "hey, it could have been a lot worse", and I was relieved that he really didn't drop the ball as bad as Schumacher did with "Batman and Robin".
Seems like people are blaming him for destroying the X-Men franchise just as much as they blame Joel Schmacher for destroying the Batman franchise, though. I think that's blowing things a bit out of proportion. As far as I know, Ratner didn't have any input on the script and he didn't make any drastic changes to the look of the film, at least keeping it visually consistent with the first two X-Men films.
The problem was more that he lacked the creativity to make it special. There's a mild competence to "X-Men: The Last Stand" that is not a given when a sequel to a good movie arrives with a new director, but that's the highest compliment I can give it, and that's not much of a compliment. I agree with the opinion that he's generic and doesn't elevate material. I guess what we're talking about then is which is the lesser of two evils - being a hack who has no creative vision (Ratner), or a hack whose creative vision is fucked up and wrong (Schumacher)?