Made for multiplayer
So, as far as single player goes, it's nothing new. If you want to buy it just for the sake of single player, take me advice, and don't buy it. You can get better graphics in M&B itself by using a few mods, and the few changes they've made in single player aren't really worth spending the money.
On the other hand, if you even once dreamed of playing M&B with your friends or fighting a realistic medieval battle with real players Warband is the only game ever made that can offer you that. This is not supposed to be bought as a single player expansion pack. It's supposed to be bought as a pack that adds a multiplayer aspect to a game that might well be considered one of the best and the most innovative games ever made. Like it's already been written, Warband's multiplayer is something you actually need to play to realize what you've been missing. No amount of appreciation can do it justice. The euphoria of beating down an opponent after a hard melee is something one simply has to experience oneself.
So, as a last word, I'd like to repeat; If you already have M&B and don't plan on playing online, don't buy Warband and then gripe about it. If you don't have M&B or want to play online, stop thinking and go buy Warband this moment. You won't regret it, that I can assure you.