A Reddit guy asked what unwritten rules there were in Warframe, this reply (not mine) made me chuckle.
Don't revive allies, they probably want that 40 energy given on respawn.
Each person is expected to guard a spawn door to speed things up. spread out your team to divide and conquer.
If you are ahead of teammates and get to an elevator, go first. time is money and your teammates with thank you.
always activate life support as soon as you get it. you want to be topped off in case of emergencies.
don't mark drops, especially rare ones. it's taking away from the magic of finding things on your own, and you don't want your teammates to lose out on it.
don't interfere with assassins. a 1 on 1 duel is most challenging and therefore fun for the person being targeted.
always kill things your teammate is trying to kill before they can. you want to keep them safe.
play music through voice chat. people like having a dj.
If you are vauban, don't use vortex. it causes a lot of lag and people don't want to shoot at something that just stays in one place and is boring. Do use bounce as much as you can, because allies are safest when they keep moving.
If you are nekros, don't use desecrate. it makes bodies disappear, and people like to keep those around for a bit longer for realism.
If you are trinity, don't bless your allies. they could be running a rage build and if you restore their shields rage won't proc.
If you are loki or valkyr, use swap teleport and ripline on allies as much as you can so that you can confuse the enemy AI
Some others :
Feel free to keep shooting at Sargus Ruk when his weakpoints are closed. You'll break through his armor eventually.
Elevators are the perfect place to use Ripline on your teammates.
When running Void Defense, remember to activate the lasers by the pod to cut down large swathes of enemies.