Each character belongs to one of six occupations, each of which comes with its own set of upgradeable perks. You pick one of three action abilities to compliment your chosen character's passive abilities (more explosive damage, better healing, etc.), and his or her specialty becomes a serious advantage. For example, the Surveillance character can sense enemies from far away, while Recon has temporary invisibility and Demolition can set trip mines and sticky bombs.
The distinctions between each build make it all the more important to play with human teammates, especially on higher difficulties. Valuable health and anti-viral sprays will be especially hard to find, but Medics and Field Scientists spawn with them. Yes, it can be a little unbalanced in versus mode—direct perks like limited-time infinite ammo always trump indirect ones, like the ability to turn zombies into temporary allies. But the breadth of differentiation they introduce to this arena is worth a little lopsidedness.
That does bring up one of those glaring faults, though: the A.I. is shamelessly, no-holds-barred dumb as rocks. Your allies will stroll through tripwires like it's their mission to get smeared on the walls, and they'll certainly never be there when you actually need some backup. They're not helped by the game's slew of technical bugs; at one point an AI teammate glitched himself off a descending elevator platform, and it was several minutes before I saw him again. Another bizarre bug (in versus, this time) formed a mirror image of my character that circled around me as I ran, firing its own gun when I shot mine. I had to quit the match for the mimic to disappear.
Snapping into cover works well, but some actions standard to cover-based shooters are absent. I understand dropping the roadie run of games like Gears of War and Mass Effect 3 (ORC's less janky sprint is better anyway), but the ability to vault over cover (or jump at all, or even drop short distances to the ground) is sorely missed at times. For some reason you don't accrue ammunition automatically, though you'll constantly be prompted with the option to pick up fallen weapons that may or may not have more ammo than what you're currently carrying. There's no way to know. Worse yet is the game's refusal to prioritize your action button commands; nothing's more frustrating than juggling weapons instead of reviving a fallen teammate.
The game's dark environments (turn that gamma up, up, up) and breach-and-clear shootouts beg for a flashlight. And unless Raccoon City's maternity wards are constantly filled with entire human litters of identical babies, there are way too few zombie character models. And—oh god, I'm a broken record—why oh why do developers continue to snub splitscreen multiplayer?