As Tale said, the difference between Battlefield and the Bad Company games is an emphasis on consoles and an actual single player campaign.
Battlefield 3 has a single player campaign, I believe, so it is breaking from the tradition there. It is still a return to the big maps with jets and a ton of players, so I'll pick it up day one without question. I could care less about a single player element, I plan on hiding in the hills and shooting as many people as I can in the head from 300 yards out.