It's another Total war game, simple as really
That said this is still a truly awesome game, the graphics are amazing, even mind-blowing in certain situations, the attention to detail is so much more apparent here than it has been on any other TW game.
The new Campaign map has also been tweaked. There are new units for use such as Cardinals and Merchants as well as actual characters for Heretics, it doesn't really affect the gameplay as much as I'd like but it's a nice touch, also Diplomats actually talk which made me smile, it gives the game a more personal feel:P
All in all this is the best TW game, no doubt, it's still the same game at heart but it has enough new stuff to make it throughly engaging and enjoyable.