The customizability of the weapons. Borderlands 2 and Halo 4 have nothing on the number of weapons and mods to accomodate any playstyle you like. I've seen people tear waves apart with Raptors, Katanas, and Geth SMGs just as often as the heavy-hitting weapons. Modding for more ammo, dmg, accuracy, armor-piercing, melee...
The variety of character classes to play. Again BL2 and Halo 4's new systems didn't hold a candle to the Power selection progression we all loved from Mass Effects 2 and 3. Even back when we just had 6 generic classes to choose from, there was a different build for everyone. Combined with the variety in weapon modding, you could play precisely how you wanted to. I've seen Adepts with sniper rifles do well, as well as concussive spam soldiers with Phaestons.
As for myself, I love the element of accuracy in the game. Taking something out with a well-timed headshot in a co-op setting helps me feel like I'm contributing... and it makes me feel (correctly or incorrectly) that I'm becoming a decent PC sniper. I'm generally not a fan of PvP, and this customizable co-op is unlike any other FPS I've tried before or since. I may hate the game's ending, and may have next to no friends still playing this game... but damn is the MP still satisfying.
.... even if you did nerf my Tactical Cloak =p