I've only tried the multiplayer beta (which is open for everyone with a valid serial key now). It's a bit like counterstrike with swords and horses. Nice variety of game modes, although some only makes sense if you have a lot of players, like siege.
It's not that different from the regular M&B battles, but there are some changes:
The most significant is probably how the attacks work. Sword swings are different, especially the forehand swing, and are a bit harder to do properly. Couched lance is only possible if you ride straight forwards (with small course deviations allowed).
A new faction has been added called the Sarranid, loosely based on historical Sassanid/persians, and there are heaps of new weapons. A particularly nasty one is the Rhodok cleaver.
Some extra graphical effects have been added, like DOF. There's also a HP gauge for the horse.
Siege ladders can be thrown down. No sign of working siege weapons unfortunately.
There are some negatives though. Network latency means you have to time attacks to hit sooner than against the computer, this is especially tricky on horseback. And the only way to be good seems to be to abuse the controls as most as possible. You'll see a lot of people jump-attacking people on horses or almost spinning around in circles. And for some reason everyone seems to be able to attack a lot faster than I can...
I can recommend a warband mp mod called Hunt, which pits a pack of werewolves against a gang of 18th century hunters armed with muskets and crossbows. It's got a really nice atmosphere and the maps are great.
Edit: Link to installer for Hunt 1.0:
http://www.mbrepository.com/file.php?id=1721