BF3 has a bunch of cool new functions such as the destruction engine, various gadgets (I especially enjoy the Stinger and Javelin missile systems), plus beautiful sounds and animations.
However, I just don't have as much fun playing the game. I think this stems from the map design being different, I think. Most of the BF2 maps were fantastic, which is evident in DICE re-releasing them in the Karkand DLC. Even in BFBC2 I started noticing the difference, as even there only 1-2 maps were truly fun to play. In BF3 I only play 2 mediocre maps.
I also don't see the need for unlocking everything that's needed to make you even slightly useful.
Let's see if BF4 brings anything new to the table, although I doubt it.