The gravity changes, the living environments, the spirit walk and deathwalk, the portals and portal boxes, are all new, fun ideas that haven't been done before. This game gets a "10" in the originality department. The weapons are very cool, and much different than the standard pistol, shotgun, machine gun, that are in most FPS's.
It may be kind of easy, but I haven't tried Cherokee mode yet, and I honestly don't like games that are really frustratingly hard. I don't want to throw my computer or console out the window in frustration (Devil May Cry 3, anyone?) I've died quite a few times in this game, a lot of the walking, flying guys, and the big guys with the guns have a lot of HP and it takes a while to take them out.
Did anyone mention how much fun this game is? It's a fun, original game with a decent story. The Native American take and spirit/deathwalk ideas that come with it are cool.