I would like to stress how important it is that you follow the order the tank marks things in when attacking single targets (unless you have a damn good reason) A tank can easily keep hate when the DPS attack his target. It's a little harder when they are using AoE, but still feasible since each mob is receiving less damage individually. However, if DPS each attack different targets with single attacks, the tank needs to gain immense enmity on multiple targets, which ain't happening. Furthermore, focusing all the damage on one target means it dies faster, making the tank and healer's jobs easier.