There will be some spoilers.
Always remember which stuff is the good stuff.
The first half of the movie is so strong because it's focused on Erik, Charles, and Sebastian Shaw. They are the strongest characters, and their relationships with each other (or, really Erik's relationships with the other two) are by far the most complex and interesting relationships in the movie. The second half is less engaging because more and more time is devoted to the newly recruited X-Men, who are not all that interesting.
The best action scene is in the first half, too (the confrontation between the Coast Guard and Shaw's yacht and how that plays out). Way more compelling than the ending (except, of course, for the final scenes between Erik and Shaw and Charles).
There's been some good discussion of this question ("what is the good stuff?") over at Bunny Ears & Bat Wings, in response to my guest post.
Some things do need to pay off.
As I talked about in the post about Chekhov's gun, not everything needs to pay off, but some things do.
Sebastian Shaw praises Emma's beauty, and then sends her out to freshen his drink (like "a good girl"). Her resentment at this is obvious, but it's never referred to again.
Two of the CIA men talk about getting information about Shaw through "a back channel," indicating that somebody on Shaw's team is really working for them, but this is never referred to again either.
What messages are you sending?
In general, and especially because this is a movie that uses mutants as a metaphor for real minorities, the final good/evil breakdown is unpleasant. Of the X-Men, who ends up on the good side of the ledger? All the generic non-ethnic white men. Who ends up evil? The Jewish guy and the two women. And, of course, the Black guy died early (google "the black guy dies first" to get some idea how predictable this is).
Think through what you're saying.
Throughout the X-Men movies, Charles and Erik debate how humanity and mutants could, or could not, co-exist. Erik always asserts that humanity will try to wipe mutants out. Charles always holds out hope. The events of the movies always show that Erik is right, but the movies never acknowledge this. I gather there will be more sequels, so maybe at some point one of the students will say something like, "Yo, professor, we like you, and we're learning a lot at your school, but, dude, you really need to wise up."
I'll throw in one more.
Sometimes the geeks are right.
Comic book enthusiasts sometimes get worked up when the movies take liberties with comic book stories and characters. I'm pretty relaxed about that, since I realize that changes need to be made, for a variety of reasons. The main thing I want is for the movies to get the characters right, even if the story-lines and relationships are altered.
That was one of the pleasures of the first X-Men movie: it got the characters right (so I didn't really mind the idiotic story). And when I heard that this movie would have Emma Frost (The White Queen), I had high hopes. Emma Frost, at her best, is one of the best characters I've ever read in a comic book. A former villain, still capable of cruelty, she is smart, sarcastic, witty, and complex. (I have a character who is somewhat inspired by her, in fact, though both are really the daughters of Angelique from Dark Shadows.) In the movie, as one critic pointed out, Emma has two characteristics. His words were "bosomy" and "sullen," and that pretty much says it.
What a lost opportunity. At one point, Emma Frost was supposed to appear in the last movie, played by Sigourney Weaver. That might have been something to see. But, of course, a strong female character like that might not have fit that well in such a boy-oriented movie as this one, where the two main female characters spend most of their time trailing along after the men. When Raven says she's been Charles's pet all those years, she's pretty much nailed it – and her solution is to switch her allegiance to Erik instead.
Which is also quite different from Mystique in the comics...
But don't get me started.