Balance over a lot of things actually: Balanced maps, balanced weapons but most importantly, balance over players.
By that i mean FAIR TEAMS and PEPOLE OF YOUR OWN SKILL LEVEL.
I've never, EVER, seen a game where I've been able to play for 3-4 days, and then find players of my own (or slightly below

This annoys me to great length, since most of the goals in the game (and ergo, the rewards) are based on winning, having a chance of winning, or at least accomplishing something. Feeling you make a difference, or do something cool like sniper some guy, or take a important objective instead of dieing horrible after 2 seconds.
And another thing that annoys me is how game producers forgets the AI...
prettier graphics, but oh, the units still cant get around a barrel and in the end gets stuck in midair thanks to some box, screwing their animation up and making them look like they're having a seizure.
Or when your fighting 2 sides at once (flood&covenant, combine&zombies) is that everyone wants to kill you. As soon as your discovered, forget about alllll the firefight and just go RAGH at the player. I can't just let them battle it out, cuz then i " miss the excitement".
BLAH, RANT! >:O