The demo covers two battles, three if you include the tutorial. The tutorial is the Battle of Hastings, and it's a good rundown for beginners. The Battle of Pavia and the Battle of Agincourt are the only other scenarios available. The second being a battle in the middle of the Hundred Years War, you command the forces of Henry V and are pitted against French cavalry, crossbowmen, and knights and whatnot. It's a really fun scenario as you can see the hundreds of men at your command clashing with the hundreds of your adversary's.
What I also really liked about this game was the use of gunpowder. Though you don't get to use any cannons yourself, and have to instead take out your opponent's, arquebuses are available at your disposal, and your Spanish allies have a few musketeers, as well. Lining up my matchlock infantry against charging soldiers tickles me in ways that I've never imagined. I'll be looking forward to more of this in the final release.
The demo doesn't cover much of anything else, however, and leaves you thirsting for more (which is the point to a demo). I did wish they included some playable material about the new diplomatic options, religious aspects, etc... but they do include a video of most of the things covered in Medieval 2, so don't you ladies fret.
Anyways, I'm really looking forward to this game, and will play it as soon as I can get my hands on it. Until then, I shall spill French and Saxon blood by the thousands at a time. FOR THE EMPEROR!
P.S.
I'm looking foward to the Crusades so much, that I think I just shit my pants.