The big problem I had was that Caspian was the central character, and the film was shown largely from his point of view. The first movie was completely from the kids' point of view, which was a much better way to present it.
I think the director spent too much time making a strong, realistic noir film and forgot it was a fantasy adventure. Thankfully, the third film is going to be directed by someone else (he directed Amazing Grace and one of the 007 films), so I think there's hope. But I think rpgdirector is right on -- the kids need to be stronger. Since Edmund is the oldest in the next film, his superior acting skills (compared to Peter's) will hopefully help.