They've done a hell of a lot right. That's my response to Singer's X-Men films; they're slick, funny, emotive and a real blast to watch. Without overdrawn character introductions and explanations, X2 launches right into seventh gear and doesn't let up.
Admirably, the film allows the majority of its characters a genuinely engaging storyline amongst the bigger picture, and although the story is not without its fault (using Cerebro as a major plot point two films in a row is like building the second Death Star in Return of the Jedi; been there, seen it before) it is certainly worthy of high praise. It's amazing that in just two films we already have a real appreciation for the relationships between such a considerable group of characters, and it is for the continued development of these relationships - and not the cool story line and action sequences
would love to say that it's the casting that makes the films so brilliant - and it's true, the casting is great - but a couple of these characters have been woefully miscast. Iceman stood out for me, bringing an uncomfortable Backstreet Boy vibe to the role, and although his role in X2 is small, James Marsden just doesn't seem to have it in him to hold a commanding and authoritative position among the X-sters as Cyclops. In the first X-film he managed to at least stand beside the Wolverine with some great gags, but he's been left in the gutter for X2, and in the single scene toward the end of the film where Marsden is allowed to genuinely act, his emotional response feels awkward and forced.
Thankfully, most of Singer's new cast are admirable. The recast Pyro is great and Nightcrawler is fantastic. In fact, Alan is the only actor in the film who comes close to matching the quality of Jackman's Wolverine and McKellen's Magneto. These wonderful actors all appear to genuinely revel in portraying their characters. Janssen steps her performance up a notch too and it helps the film no end.
Ultimately, there are just a few slip-ups here in an otherwise classy and supremely entertaining film. With such an established canon of characters everyone is going to have their own opinion on how particular character should be portrayed, and for the most part X-2 does the job...