Basic stuff:
Medics are invaluable. Make sure your team's medic stays alive, and the enemy's medic stays dead, by any means necessary. If you die so your medic lives or the enemy's medic dies, you've done your job.
Engineer "nests" (sentry, dispenser and teleport) are second only to good medics. Same as above applies, with the difference that nests don't move, and they're extremely vulnerable without an engineer around. Get the heat off your buildings if they're under attack, even if it means taking a bullet or ten for them. Conversely, make sure the enemy buildings turn to rubble asap... This is made easier if you first turn the engineer himself to rubble... Be imaginative.
A team that slays together, stays together. 2 guns kill better than one. Moving as a team assures the enemy has more targets to shoot from, and more fire to avoid. I.e: Less damage for everyone, everyone stays alive longer.
Learn your class, and for the love of god don't be stupid... I can sympathize that people won't immediately know the intricacies of each class as they start, but some shit is just obvious... Sitting back and camping just lets your enemies carefully mount an offense. Keep pressure on your enemies even if you're only defending. The objective comes first and above all, kills mean shit if your team looses the objective. If your team has 5 support classes already they don't need a 6th... Etc. Overall, just think for a second.
Goodluck.
EDIT: The following unlocks are junk, regardless of other issues, and shouldn't be used if you plan on not sucking: The Backburner, The Sandman, The Huntsman, The Razorback.
Some of the other unlocks aren't very good, others are great, but the above shouldn't be used for any given reason except noobing it out.