To add to Jeff's answer: Be useful...
If you are a medic, be liberal with handing out medikits. Whenever you dash and stop, leave a medikit. See a team mate, drop a medikit.
Same with support: leave plenty of ammo boxes. For some of the urban maps, grenades become very useful so your team mates will like you when you replenish them.
Don't run around aimlessly. If in doubt, find cover and look about for baddies and try to spot them. If you run around 'looking for action' then you WILL get shot. Or knifed. Or RPG'd!
If your team arm the MCOM, then don't just stand there looking at it! Turn around and look out for the defenders coming to disarm it. If your team mate looks left, then look right and cover his six. Scan the rooflines. Spot spot and spot.
If you have a headset (when it works) then don't speak drivel and yammer on and on.
If you get into a tank or vehicle as the second passenger then use the machine gun to spot, to kill engineers and recon's with a handful C4 running up to toast you. Don't look forwards (the same way the driver is driving) but look-out to the left and right and behind and up (where the driver can't see) and take out infantry, choppers, jets etc.
Oh, and remember to blink! Your eye's can get very dry and sore after a couple of good games!