Unless you are intending on spending your entire time in WoW running solo, you need to know how to deal with and react to team members and group play.
Despite what a lot of PUG (Pick-up group) members would have you believe - your damage is not the be all and end all of running in a group. In fact unless it is very bad, it doesn't really matter at all what your damage is. The main priority of ANY member of a group is to know what you can do, and when you should do it.
The three main roles in any group are Tank, Healer, DPS (Damage per Second).
The Tank is primarily there to be hit - to soak up the damage and keep the attention of everything on him.
The Healer is primarily there to keep people alive - priority being the tank, then the healer, then the dps (and if they want to prioritise the dps, the most likely to help finish the encounter without dying gets healed first).
The DPS is there to do everything else.
Each role is hugely important and are broken down into more detail below.
The tank has one of the more difficult jobs in a group.
They are looked upon to know the encounter inside out, to be able to mentor the other members of the group, to be able to mark a kill order (and therefore know what each mob does, how he does it and how dangerous it is).
They have to have enough of the right types of stats to soak up damage and have the situational awareness to know when to pop those cooldowns to temporarily boost their health or ability to avoid damage.
They have to keep an eye on all of the current group that you are attacking and know what each mob is attacking, and if any are about to peel off to attack someone else (or run away).
They have to watch any crowd controlled mobs to ensure their crowd control doesn't break or, if it does break, gain control of them quickly.
They have to watch for any other groups of mobs close by, and for any mobile group that may get close, to ensure they are not pulled accidentally (and if they are, attempt to control them quickly).
They have to be aware of all other members of their party - know where they are, if they are in danger, if they are out of mana (especially the healer) and to compensate.
They have to know when a particular encounter should start - taking into account the parties health, mana status, buff status, readiness, etc - and how to start it (line of sight pulls, charging in, pulling back from another group).
They have to know placement of an encounter - if it involves moving in a specific way, if the party should be positioned with their backs to a wall, in a pool, move at certain times, stay behind, or to the side.
The healer often has one of the most unappreciated jobs in a group - especially if it is a PUG. The only time they (or indeed the tank) are acknowledged, are if people die.
Unfortunately, despite the huge amount of healing abilities available to a healer, the one thing a healer is unable to do is "heal stupid".
The vast majority of the time when a member of a group dies, or everyone dies, it is because someone did something wrong. Often it is accidental, but just as often it could be avoided.
The healer generally stands as far back from the encounter as possible to reduce threat radius (the closer you are to a mob, the higher the threat you generate), but ensures they aren't likely to pull a seperate group from elsewhere. They have to be situationally aware enough to be able to keep out of fire, stay away from any incoming damage or patrolling mobs, yet still be close enough to heal members of the party.
A lot of healers are not able to heal well whilst moving, so they also have to be able to time their heals in between any required movements.
They have to be aware of the threat they are generating with their spells and ensure that party members are healed up prior to new encounters, yet ensure that healing is not occurring at the start of a pull - this will generate threat on all mobs in a group other than those that have been tagged directly by another member of the party.
Healers are often able to remove various debuffs as well - they need to be aware of these, and how they affect the health and healing of the person with the debuff. They have to decide whether or not spending the second or so debuffing and not healing is worth it.
Healers are also expected to anticipate healing needed. This is more difficult on an unknown encounter and/or with an unknown group of people. If they know the encounter, they will be able to better anticipate when the larger amounts of damage will happen and to whom - if they know the group of people they will have a better idea of who tends to take more damage (or who has less health/mitigation/situational awareness) and is therefore more likely to die quickly.
On more difficult-to-heal encounters, the healer has to make decisions as to who should get the heals first (and who is getting the heals last). Generally the first to receive less healing will be the dps character who takes more effort to keep alive - once they are dead, then all that time and effort can be spent keeping the rest of the group alive. This may not necessarily be an actual player-character - it could be a pet.
Healers should also be aware that pets are often a substantial amount of a hunters/warlocks damage and that they need healing too. Indeed in a tricky situation a pet can often temporarily keep a mob occupied and away from the rest of the party.
The rest of the group is made up of the "DPS". To be honest, I find this to be a bit of a misnomer, as those remaining party members should not think of themselves as only damage dealers. Yes - at the end of the day taking down the mobs using damage is the way to go, but much more is involved in this role.
The DPS must be highly aware of their placement. Melee especially must always try to be BEHIND a mob. Not only do many mobs do cleave type effects which will damage everyone in front of them, but also if the mob avoids one of your attacks (which is much more likely when you are face to face with them), then there is a possibility that it will do a higher than normal damaging attack on the tank (or you!). The only exception to this, is if the mob is a dragon. Generally only the sides of a dragon are "safe" (they like to swish their tails!).
DPS must always take their lead from the tank - unless the tank requests it, they must NOT pull mobs. If a group of mobs are marked, the dps should always start attacking the one marked with the skull first, then the cross. Other markers may be used to indicate certain types of crowd control which, again, the dps must handle.
DPS should only use AoE (Area of Effect) spells if there is no possibility of pulling an extra group of mobs, and no possibility of breaking any crowd control. They should never "open" with AoE as this will likely increase their threat on some of the mobs faster than the tank.
DPS (and healers) should be fully aware of any talents to reduce threat, or to transfer it elsewhere, and be ready to use it should they gain threat.
DPS should know how to crowd control, and when to apply/reapply it. They should also be aware of their healer - they should try to stay in range of the healer (if the healer has to run after you to heal you, you're much more likely to die), and ensure the healer is protected.
If a DPS pulls threat away from the tank - 9 times out of 10 this is the DPS's fault. Unless the tank is asleep or not properly equipped, there is no reason why a DPS cannot keep their threat under that of the tanks. Other than ego size. It is always easier for a dps to keep their threat UNDER that of the tanks, than for a tank to keep their threat OVER that of the DPS.
DPS should be aware of how often the tank is having to use their taunt cooldown - if taunt isn't available, the dps that pulled suddenly becomes the tank. As mentioned in the healer section ... healers can't heal stupid!
At the end of the day, a group that does less damage, but knows their class and abilities well, are much more likely to survive, and have fun, than a group that simply goes all out to top the damage meter. High DPS and Gearscore do NOT prove you are a good player. It just means you have grouped with them! The high DPS and Gearscore will come completely "accidentally", with a whole lot less repair bills, if you can become a good team member.