Aside from general gameplay/mechanics which are reasonably similar in both games, the main thing for me was atmosphere.
BF2 is set in a world we can relate to, it is our world, it has an atmosphere that we can believe in and embrace. 2142 is set in a make believe future which relies heavily on out imaginations to make it believable, and therefore enhance game play. This is where 2142 falls down.
It's too similar to most other 'sci-fi' shooters and lacks, IMHO, any real atmosphere. Now this is personal preference, but prefer my sci-fi gritty and dark, think the original Alien and Event Horizon. I am a big fan of W40k universe and I would love to play a FPS based on that, but 2142 just seemed to me to be guys just wearing different clothes with lazer guns and the occasional 'future' tank type doodad.
my 2c