Creating a class is probably the best unlock, at least in the beta, letting you select which perks you want. This really opens the game up to many different options, and really lets you play how you want according to the situation at hand. Have an enemy that hides behind cover? Then pick someone that shoots through walls. Does your enemy turtle as a pack in a small room? Pick a class that carries multiple stun grenades and the last stand ability, so you can flash and clear and take the survivors to hell with you. It almost becomes a game of chess, with you constantly countering the other team's tactics while trying to outsmart them. These perks really add a whole strategic element to the game, and force you to quickly adapt and change your style mid-game.