McAvoy is a good pick to replace Patrick Stewart, but he's awfully young to be teaching anyone. But remember, this isn't really about the first class of X-Men the way we thought it would be. X-Men: First Class would be an origin story for Professor X and Magneto. In this film, young Charles Xavier and Erik Lensherr will discover their powers, hone them, and take on their cool aliases. They will then join with fellow mutants "some familiar, some new" to stop a global threat. Their friendship fractures, and creates the rift between Magneto's antagonistic Brotherhood of Mutants and Professor X's peaceful X-Men.
It's unclear who these mutants will be. (Beyond a Wolverine cameo, of course.) Are they actually going to be young Jean Gray, Cyclops, and Storm? If so, it's nice to see Fox sticking to their resolve of continuing to throw their X-Men continuity out the window. You can't exactly have a movie featuring young Jean Grey or Cyclops, if the Professor X and Magneto we saw interacting with them (in X-Men: The Last Stand and X-Men Origins: Wolverine, respectively) were the older and wiser figures of Patrick Stewart and Ian McKellen. Perhaps they're going to haul out those amnesia bullets again to explain the discrepancies.