X2 ended with the 'death' of Jean Grey. While this results in a complex (and highly acclaimed) resurrection arc, 'The Dark Phoenix Saga,' it isn't likely to be completely explored in the new film.
Presumably, Jean's death traumatises Cyclops, after which he leaves the X-Men, which is where Storm steps in as leader.
Another notable addition to the X-Men this time is Beast. A brilliant Irish-American biochemist and political activist, this pacifist (right) finds himself ironically trapped in the skin of a cobalt primate. With ape-like dexterity and a remarkable intellect, this articulate and misunderstood genius is played by Kelsey Grammer.