It adds a bit of new stuff.
A multiplayer mode (which is surprisingly great aside from being overrun with assholes), a few new navigation abilities (although if you played bonfire of the vanitites one's only sorta newish), a reworked combat system (which makes Ezio hilariously overpowered right from the get-go) and the largest being a sim game where you train/command an army of assassins. Which also ends up giving you an
obscenely broken special ability.
The plot is a lot more linear and smaller scale this time around but I felt they did a good job of it. It's pretty much an expansion on AC2, but it's a damn fine expansion. I enjoyed it. Not nearly as much as 2 (or potentially a real AC3), but it was fun times. Oh, and they finally got around to making Desmond do something besides sit around on his fat ass all day. So there's that.
Although Ezio has so much bullshit at his disposal by the end that you're taking down mobs of elite guards like they're an army of bards. Hell having them bumrush you (which they will) just makes you wonder whether to call in a tactical nuke strike, bounce around from one to the other like some sort of murderous blade covered pinball, or simply turn them into your unwilling puppets. So if you're looking for super difficult gameplay, Brotherhood isn't it.
