Make sure you know the loot rules before you start.
If you are in a PUG, be sure you know the loot rules beforehand. Also don’t be greedy. If you have gotten some nice loot Pass and let others have a chance. Also you need to know what your character and build needs equipment wise before you roll on that item that might not really be suited for you.
Primary Roles
The most important aspect of group play is that all group members know without doubt which of the three following roles they play, and put this primary function above everything else. Confusing these roles leads to disaster - a tank (or healer) who believe that they're damage dealers are immediate causes for wipes. Similarly, the damage dealers must be clearly aware that they're not tanks, and thus avoid getting aggro by all means.
Healers do no damage at all - even if this means they have nearly nothing to do at all on some pulls. They save their mana to keep the tank and everybody else alive. Healers can die from their heals drawing aggro. When they have drawn more aggro than the tank they have to stop healing everybody else except the tank. If the healer still dies, then the tank is either not geared enough to hold aggro or must do a better job at holding it.
The tank is the group member who should take all the damage. Tanks don't deal (much) damage themselves, their job is to gain and hold aggro on all mobs (a.k.a. enemies), and make sure nobody else is hit. A tanks primary task is to ensure survival of the healer.
Every class has the ability, with the appropriate talent points and gear, to deal significant damage. Without sufficient damage from the group, it is likely that the healer's mana runs out or the progress through the location is too slow to be practical, whether for Respawn or other reasons. A good damage dealer knows how to kill mobs without drawing aggro, a bad damage dealer doesn't and dies. The central and most important rule for damage dealers is "dead = 0 DPS". If a damage dealer dies, it's not the healers fault, neither the tanks, but always the damage dealers own. Stop DPS and bandage if everything else fails
Secondary Roles
After the above three functions are well understood and practiced, proper understanding and handling the following functions further increases the partys efficiency:
The Leader (mark mobs with Raid Target Icons) - selects targets for Crowd Control, damage, and off-tanking or kiting
Puller (start the fight with the first hit on the mob) - In groups of five this job of pulling is usually done by the tank himself. If available, a good Hunter can substantially increase the groups survivability in difficult pulls though (and since a good hunter is a bad hunter with a lot of practice, it's a good idea to let hunters do all the pulling all of the time)
Crowd Control (CC, take mobs temporarily out of the fight) - Often hinges on the type of the mob. Polymorph is the most popular since most mobs are humanoids or beasts, but there are many other forms of CC, such as Traps, Sap, Fear, etc.
Off-tank (OT, tanks extra mobs) - on some pulls, it's necessary to assign an extra tank to a mob, either because it's too difficult for the MT to build aggro on several mobs, or because he would take too much damage, and/or there's no CC available