Evan: PlanetSide 2
Fighting against two factions instead of one activates some dormant area of my FPS brain. It's not simply that you have more and differently-colored soldiers to shoot at, but you experience this novel feeling of competition over resources that matter. No other shooter gives me that sensation. Occupying someone else's base means something—just by contending for an outpost, you're earning a tiny trickle of resources. Own it, and that earned-over-time allowance extends to your whole empire (while being denied to the enemy). The magic of that mechanic is apparent even in an hour-long play session with a character I'll never use again in a crowded, loud convention center. Whether you like it or not, you're a part of something.
Beyond that, PlanetSide 2 is better-looking than Tribes, Battlefield 3, Call of Duty: Black Ops 2, and (save for Natural Selection 2, which I love the look of) any other multiplayer shooter I can name. The sci-fi context has let SOE's art team run wild with neon and high-contrast player and vehicle skins in a way Battlefield can't. It's a miracle that this will be free.