I haven't played it yet but from what I've read the five reasons are
1: It's next-gen Prince of Persia!
2: When a level is cleared of evil the level has not been completed yet (replay factor)
3: The new cell-shading looks awesome
4: The new control scheme has simplified the game for the most part, but at the same time it has also made the game more intense
5: There are no more group battles; everything is now one-on-one, making each fight that more epic and difficult.
And a sixth, when in doubt, see reason number one again.
I know the second I have money for myself I'm picking this baby up, it looks too amazing to not touch it. I believe that if you are a fan of the PoP series it will be a definite buy.