All the above is just opinion.
BF3 has far more attention to detail, a much better engine and the infantry combat is vastly improved. The position and quality of the weapon models, animations, textures, lighting and little things like being able to look down and see your legs make it feel right.
They spent a lot of time working on this and it shows.
It contains the same rush game mode as BC2 but with more variety in terms of a mix of vehicle and tight infantry maps. 64 player conquest is catered for but the star of the show in that regard is the excellent Back to Karkand expansion which is well worth picking up.
To dismiss BF3 based on the above comments would be a disservice to yourself as a gamer.
Who the hell cares if they have "detail" if the basic combat itself is a massive departure from the perfect balance of BF2.
Everyone in the entire game in BF3 has good medium to long range reach because everyone gets scopes and everyone has a low recoil weapons that have good accuracy afar. That's bad game design.
Infantry combat feels like Call of Duty on a larger scale. There is barely any penalty to dying, there is no disadvanage to not working with your squad, as you can spawn on anyone, no Commander, no assets to blow up, no uncaps to sneak around in wrecking havok, no inbuilt VOIP, they took about key weapons like the AT4, most of the maps are tiny, and the only maps that are even really worth playing are the ones that they copied from BF2, and even then they neutered those maps to be more casual.
Bad Company 2 at the very least, I didn't expect much from. While it didn't have all of the above from BF2, what I got was a game that has maps even bigger than most of BF3's maps, pretty damn decent balance, extremely fun mechanics such as tagging with the dart and shooting a rocket at him, a great feel to the destruction because most everyone was demolishable, and fun gameplay because at the very least combat had a decent about of flow to it. There isn't a single BF3 map thats as interesting as the desert MAP in BC2 with the criss-crossing through stranded ships or the city map with the giant port in the middle. BC2 had heart in it. BF3 is a sterile money grabber.
BF3 is a damn stunt by EA to get at the Call of Duty crowd. And it shows when the Demo is one of the worst maps in the history of shooters, and the first expansion is Call of Duty: Battleroom, with only close combat, and straight up "Domination" gametype from COD.