Well, there's not a whole lot of variance you can do when it comes to a first person shooter set in current times. DICE said they wanted to make it feel like you aren't some superhuman hero, just an ordinary soldier. In my opinion, they achieved that (especially on hard). Also for variance you get to drive a tank, shoot lots of stuff with it, and be the second person in a jet (shooting and popping flares)
With that said, it is still an FPS set in modern times. Also, it's apparently quite short and I wouldn't doubt that. I think I have two levels left and I've only been playing for 6 or so hours. Definitely play on hard if you want to feel that "ordinary soldier" feeling.
Battlefield has been and will always be a multiplayer oriented game. In my opinion, singleplayer is something DICE tacks on to give a more complete experience. (In BC1, BC2 and BF3 I can say that. Although, the squad of four in Bad Company were a great idea)
Took me 8hrs to beat the campaign mode first time around. A lot of twitch combat, and pressing certain keys during cutscenes. I died a lot during the cutscenes cause I wasn't quick enough to press a key.