411mania - The biggest addition is the giants themselves, of which eight are planned right now. The giants still have one of the eight elements associated with them, but are much bigger in scope than the normal characters.
They also had quite an easier time taking on enemies than the normal Skylanders. Their attacks are stronger and, like in the last game, can be leveled up. In addition, they'll also have some specific puzzles and ways to interact with the world while you progress, like pulling an island closer to walk on.