2142 was the most interesting BF game imo. There were things the game did excelled in, and also ways that it failed miserably. So much so that they basically cancelled each other out.
What the game did wrong :
a) the game was the buggiest out of all of them. Even today, if you play it, the movement is sluggish, buggy, etc. The hit detection is also pretty shitty. Especially on titans, of course, the game becomes unplayable.
b) the game had probably the worst vehicle combat of all the Battlefields to date. The ground vehicles, aircraft, etc. All of them were just fucking boring, hard to control, etc etc. Just horrible vehicle gameplay.
c) it was the first Battlefield that only featured two main factions... an ominous sign of things to come
What 2142 got right
a) I'd argue that the classes were fairly well balanced, and the infantry combat in general was pretty superb (which is ironic considering the bugginess of the game). Squad leader items were great, and just the general layout of the kit selection screen was the best there was. Simply click and drag items, heavy armor vs. light armor, etc. etc.
b) I enjoyed the setting and the risks they took with the game. Titan was a bold, if flawed, idea. I liked the implementation of futuristic equipment, such as the rocket pods. I loved the futuristic architecture of the buildings, the general layout of the maps, etc. It was a much more vertical game because almost every tall building was accessible in one way or another.
c) I thought Northern strike was a really great DLC, in my book it ranks up there with Special Forces and BC2 Vietnam. I loved the game mode where you had to take out the enemies armored vehicle. I played Northern Strike as much if not more than the vanilla game.
Conclusion : 2142 was the transition of Battlefield from large scale vehicle combat to close quarters, infantry focused gameplay. (Giving the engineer an anti-aircraft rocket, for example) Interestingly enough, I'd say its also the closest to BF3, in the way that the lines between kits are blurred by each having a selection of different types of weapons (recon gets a sniper or carbine). The difference being, 2142 still had a traditional BF, PC mentality -- you could generally go anywhere you want at any time you wanted. If there was a place that was out of bounds, they'd put up a wall or a fence to let you know. It was basically BF3 gameplay, set in the future, with better maps.
Hope you enjoyed.