A fun origin story set in the 1960s, X-Men: First Class sets out to accomplish what many comic-book films fail to even consider: that of delivering a complete story. Unfortunately, it tries to pack so much story into the span of 132 minutes that its exploration of thematic elements borders on schizophrenia. The haphazard nature of its development serves the distracted generation well, yet leaves those yearning for more complete stories feeling as if something is missing.
The multiple personalities of this particular storymind focus on two stories: the central one pitting Main Character Magneto (Frankenstein's monster) against the always-positive Influence Character Prof. Xavier, and the secondary sub-story centering around Mystique's sad-blue monster as Main Character. In this smaller story, Magneto fulfills the role as the Steadfast Influence Character who eventually convinces Mystique to change her resolve. It is precisely this dynamic, however, that weakens the central argument of the first story. For that smaller story to work, Magneto's position on the goodhearted nature of men must already be set. It can't really be an argument for Prof. X and he to bat around. As a result the presence of this secondary substory makes the argument in the first a foregone conclusion.
In other other words--twenty minutes in, the audience instinctively knows how things are ultimately going to turn out and the film becomes an exercise in Oh yeah, how can you thrill me next?
Now, the film certainly does a damn good job of answering that question
True, because of its purpose as an origin story, X-Men: First Class starts with a heavy disadvantage in the surprise department, yet there can still be an unknown quality to how an argument plays out regardless of the familiarity with characters and setting. Batman Begins and the first Spider-Man managed to do this. No reason First Class could have been added to that list. Lose the substory with Mystique (and the extension of that argument with Beast) and the film's thematic issues would have seemed less chaotic.