See, I never played BF2 so what differentiates this from Bad Company? Are we expected to see bigger maps? Different classes? Different Modes?
The maps in Battlefield 2 were much bigger than Bad Company 2's, therefore there was more players (64) and the squads were larger (6), with a squad leader that would forward on mission objectives from the Commander. The Commander was a player that would hang back in the base and sometimes fight (but this would be frowned upon). The Commander would issue objectives and deploy airstrikes and supply drops.
There were more also more armies featured and therefore there was a bigger selection of weapons. There were also more classes (Assault, Engineer, Medic, Spec Ops, Sniper, Support and Anti-Tank), but they were merged together for Battlefield 2142 and the Bad Company games.
Conquest was also the only game mode, I think. I'm not sure if another got added, it's been a long time since I last played.
It's hard to tell what Battlefield 3 will be like, but I'm hoping it's bigger and better than Battlefield 2 and not just Battlefield: Bad Company 2 v1.2.Edit:
And of all the maps they had to remake: Bloody Karkand!