Cap'n John said in a previous comment that the DPS should listen to instructions and know how to perform their role, so I thought I would make a post about it.
DPS encompasses pretty much every class in World of Warcraft; that is, even warriors and priests have the ability to perform some amount of DPS. Due to this, it is very rare that a request for a DPS member will take long to fulfill for an instance.
So why are DPS so bad in instances?
Well, for a start, the game allows for solo play, suggesting that group play for DPS is actually a bad idea. In a previous MMO, I was a healer, and I was regularly picked up for group quests because it was more efficient to kill things with other people than it was by myself, or them by themselves. Monsters were very powerful as compared to a healer or a tank, and DPS classes did not have enough health or armour to keep themselves alive long enough to kill the monster - they NEEDED a tank or a healer, and we needed someone to kill for us.
In World of Warcraft, you don't NEED a tank or a healer to kill normal monsters. And you can avoid group quests all the way up to the end cap, level 70. At which point, suddenly you DO need a tank and a healer.
DPS classes have low to medium health, low to medium armour and high Damage Per Second.
This means they are able to solo more efficiently than a tank or a healer so long as they do not encounter more than 1 monster at a time. Two or three, and they won't survive the fight (meanwhile tanks and healers may survive but the fight will go on for a very long time, as their DPS is low in comparison, potentially causing more monsters to join in). In fact, solo grinding is actually more profitable than killing things in a team, in terms of speed and experience points.
Thus, DPS players do not have any experience with the role that a Tank or a Healer should bring to the group. Equally, Tanks and Healers know that something is very wrong when DPS players either take aggro away from the tank or force healers to switch to them rather than healing the tank. Many a time, DPS players will say things like "L2Play Tank! Keep Aggro!" with a swear word thrown in when the monsters turn away from the tank and bash up the DPS player. My response to this is and has always been:
Any DPS Class should be able to steal aggro from the Tank. The skill is in not doing it.