This post contains spoilers after the break. Adding just a few minutes at the climax, without changing anything else in the movie, would have made it much better. Here’s what I would have done differently:
1. Show us the threat to the entire world — show us Jean’s power growing and smashing things in London, in Tokyo, etc. Oh, and SHOW THE PHOENIX SPECIAL EFFECTS AROUND JEAN!
2. The X-Men are a team. Show the entire team trying to stop Phoenix. Maybe even get Pyro and some of the other bad mutants to help when they realize that Phoenix is a threat to them, too. But Phoenix is so powerful that she whittles the team down.
3. Fortunately, the constant barrage has weakened Phoenix, and Jean Grey is able to reassert control over her powers. She breaks off the attack and asks to be injected with the “cure” before the Phoenix power can take control again. She’s injected. The battle is over.
4. The X-Men concentrate on cleanup. Logan comforts Jean as she weeps over what she did as Phoenix. Suddenly she gasps — she can feel the Phoenix power returning within her. Obviously, some mutants are so powerful that the cure isn’t permanent. She struggles to fight it, but realizes she can’t win. She begs Logan to kill her, and after hesitating almost too long, he does.
If the climax had gone that way, I would have come out of the movie satisfied, even if I had quibbles with the rest.