There is a major difference in this game. Like Ruby/Sapphire and Emerald, This game you may capture both Palkia and Dialga instead of having to buy both games. It also has added many more pokemon to the Sinnoh Dex, making it more difficult to get to the National Dex. Whats really cool about this is the whole ideals of the Torn World, or as the movie called it, "The Reverse World" and having Giratina's origin form. Its really looking to be an awesome game, much better than Diamond and Pearl was. There are more differences but I don't want to spoil them :P
Yo.