I should stop myself from getting too hyped for this game. I don't want my expectations to be as high as my hopes, less I be disappointed when the game is released. Regardless, I can't wait to stress my PC when Battlefield 3 comes out.
The main line Battlefield games had (pretty dumb) bots for single player content before the Bad Company games. You could always set up a comp-stomp session, if you so choose. I've played a bit of the Bad Company 1 campaign and it really made use of the Battlefield pillars of infantry and vehicle combat on across a large area. It was a big disappointment that Bad Company 2's single player campaign was much more corridor heavy yet lacking in CoD-esque crazy spectacle. Still, I thought it was a fairly entertaining romp from a developer known more for crazy big competitive multiplayer. Battlefield 3 will be the first time the main line of games includes a hefty single-player component. The early footage released thus far suggests script-heavy, corridor shooting, but that doesn't necessarily have to be a bad thing. I would really love it if later levels open up more. There's a desert mission in Bad Company 2 that has you picking the order of your target destinations. It was essentially picking which corridor shooting gallery you would like to go through first, but I felt it really showed the scale Battlefield can play at.
The first two Bad Company campaigns didn't have cooperative play despite most of the levels has you with three other squad mates. There was co-op DLC released for Bad Company 2 called Onslaught, but that was just a Horde/enemy wave mode from what I've read about it. A co-op campaign in Battlefield 3 would be great but I don't expect it to be in the game. That said, fingers crossed!
Lastly, a small detail: The last reload in the trailer has the player not chambering in the new magazine and leaving an extra bullet in the rifle. This took a long time coming since even Bad Company 2 has you chambering a new round in to your firearm every time you reload a half-spent magazine even though a bullet from the old magazine should (realistically) still be in the gun.