I've played most of the bf games since the original Vietnam. I felt they all generaly improved as they went along, and I got amazing value from them. With the following exceptions:
-getting rid of hit music, and the ability to add your own soundtrack ingame, from Vietnam. Playing real music from chopper or jeep was awesome. Cocky players could play it like the red Barron painted the circus.
-torching automatic teamspeak from bf2. Added so much to the game. Really missed it.
-shrinking servers to 32max in bfbc2.
-more of a preference really, but I prefer nice epic beautiful scenery like beaches and tropical jungle, to crawling through garbage piles with flies. Feel like I'm on vacation I guess.
Other than that, many many amazing improvements. Haven't played bf3 very much to know if there are many improvements over bfbc2, other than the typical graphics engines etc.
I'm pretty picky about games, don't play anything but bf anymore.