Common sense when positioning monsters

[TouchandFeel]

If you are new to a dungeon, a role, or to the MMO genre in general, don't be afraid to let people know. Sure you'll get jerks that drop out and stuff like that, but better to get that out of the way upfront instead of waiting until you do something they consider wrong and have them flip out even more and then drop. Simply jerks will be jerks no matter what, but assuming that everyone is a jerk and won't be patient or helpful is not the way to go. There are many players out there that will be patient and helpful, as long as you don't get all defensive or refuse to listen.

Don't expect to be super elite and the best tank, or whatever role, right off the bat, because guess what your not going to be. You'll need to learn and become comfortable with all the mechanics and nuances of your role and job, and the only way your going to do that is through playing it and *shudder* making mistakes.
On the other hand, people should realize that some people may be new or not familiar with things in the game and that a simple politely phrased suggestion will often yield better results and save more time than raging like a baby that needs its diaper changed.

As far as mobs and positioning, you pretty much always want to face the mobs away from the rest of the party. This if for two reasons, first most mobs that have AoE attacks have forward facing cones, so that way their attacks will be pointed away from the rest of the group and they will not have to stop what they are doing and move out of the way, second because the melee dps do more damage when attacking an enemy from behind and the sides.
The easiest way to do this that works with most trash pulls is to run towards the group and shield lob. They will agro and move towards you. Run through them and then immediately turn around so that you are now facing them and the rest of your party. The pack of enemies should now be between you and the rest of your party with them all facing towards you and with their backs to the rest of the party. If you can, try using flash while your are running through them to help get initial group agro.
If there are ranged enemies in the group run to them when you run through the others and the melee enemies will follow you. If the ranged enemies are near other enemies that you don't want to agro, then run back until the ranged enemies begin to follow and then once they are in a safe spot to engage, turn and charge them. The best way to get ranged enemies to follow you is to break line of sight by hiding behind something or turning a corner, this is especially useful with all the ranged enemies in Darkhold.

Don't run around doing circles around enemies or other such nonsense, it doesn't make the enemies miss you any more and all it does is make it harder for the dps to do their job. Every move that a tank makes should have a good reason behind it (positioning enemies or avoiding an AoE) and for the most part you want to stay in one spot as best as possible. You can also dodge AoE's without turning or moving a mob fairly easily. As soon as the telegraphed floor marker appears move out of it and as soon as it disappears move back, even if it appears that the attack has not gone off yet. Once the telegraph marker is gone, who the attack is going to hit has already been registered by the server. This way you are able to move back into position while the enemy is doing the attack animation and they end up not moving or turning.

The more familiar you become with different enemies and the different dungeons, the more familiar you'll become with how to position them and where to do so.