It's clumsily written. Plot hints are painfully sledge hammered in while new ideas are dropped in with no explanation exactly what is a mutant mark or class? The plot is a little holey, particularly at the finale, which happens far too soon, just over an hour in. To give this idea the attention it needs, and to handle returning characters and new characters sufficiently, this needed to be much longer than an hour and a half.
That's the big problem. The characters. The Last Stand takes characters with little opinion on the cure as leads, sidelining those who do have opinions, such as Rogue, Mystique or Angel. There are too many new characters, who while occasionally interesting, needed to have been set-up in the previous films or left out. No character parallels play out and too many characters die in this film, considering the sparse death list in the previous films is it trying to make up for lost time?
There are serious character issues here, not least almost everyone suddenly being unlikeable, especially Storm, Magneto, Professor X, Bobby and Jean. Leader of the heroes, Professor X, follows the same character arc as in the first two films, but here it isn't handled as skilfully. Leader of the villains, Magneto, having long given up being a sympathetic villain is utterly repulsive and extremely irritatingly played by Ian McKellen, and despite egging Jean on constantly to kill people, still comes out with the worst-delivered line of the film, "What have I done?"
Several main characters clog up the plot and would have been better cut, making room for relevant subplots. The pseudo-romance between Bobby and Kitty is a misstep, seemingly only existing to drive Rogue out of the film, despite her being infinitely more interesting and likable than these two kids, likewise, the desperate 'romance' between Wolverine and Jean is still being paraded around, despite there being no explicable or discernible attraction between them. It is tedious that Wolverine is still desperate to get it on with Jean, despite her now being a completely different person. She is insane, has no personality, and would have been better left at the bottom of the lake at the end of the last film. She is a non-entity of a character, reduced to a psychotic weapon. Whenever the camera pans over to her, it is a jarring reminder that she is even in the film, and when she decides to actually be involved in the finale, this leads to the biggest let down of the film. There are better parallels and subplots that could have culminated here, and it is too easy for the big ending.
Meanwhile, other main characters such as Cyclops, Mystique and Rogue are heartlessly tossed aside both by their friends and the script, and their interesting subplots axed, when these are the strands that should be carrying the film.
Admittedly, new additions Beast and Juggernaut are a lot of fun but however cool six X-Men facing the onslaught of hundreds is, we barely know this small line of underdeveloped, boring characters. And is this really the last stand? Isn't this actually the first stand? It's not even that important.
As usual, my biggest gripe is that Rogue doesn't get to shine, when she has the most potential. When Rogue professes interest in getting cured, Storm claims that there is nothing wrong with her. Rogue's response should have been to touch Storm's face, and see if Storm still thought there was nothing wrong with Rogue, now that she was dead.
Stick to X-Men, and pretend it had no sequels.