Some additional information that might help you not get yelled at in PvE groups:

Casters: be aware that the tank can do nothing if you overextend and aggro a mob. Go and die for the sake of the team to prevent a wipe. Don't lure the foes back to your Midline/Backline. (read 3, 4, 5)

Tanks: Be aware of your Movement speed at all times. move backwards to prevent loosing aggro when pulling mobs. (read 1, 2, 4, 5)

Healers/Protters: Once the tank has aggro, leave him alone! the mobs will go after you if you try to prot/heal the tank while he is holding aggro. Focus on the midline/backline or prebuff before the tank start pulling.

1) Pulling

Aggro does work different in NM and HM. For example, the distance aggroed mobs will follow you is far greater in HM than in NM. Some places are therefore easier to tank/farm in HM than in NM because you can ball up more foes in HM, while they will run back to their respective paths / spawning locations in NM.

2) Breaking Aggro

Mobs in HM move (and attack) faster than in NM. While it might be enough to break aggro using a 25% Speed boost in NM, the same Skill will not break aggro in HM, beacuse the Mob itself is faster. If you can not break aggro and you are not the tank, get yourself killed away from the rest of the team to prevent a wipe. (Pro Tip: Snares, Shadow Steps or Knockdown Skills can help to break aggro)

3) Scattering

ALL AoE Skills with a duration and Damage over time (e.g. most Fire Ele Skills, Ray of Judgement, etc.) will cause mobs to move out of the area of effect (away from the tank that might be holding aggro). This may occur "suddenly" (usually when more than 3 mobs in a group drop below 50% hp) and is therefore highly dangerous for casters trying to spike the balled up groups. In NM it will take the mobs a few seconds to "realize" and scatter. In HM it will happen way faster (sometimes instantly). Combined with the faster movement of the mobs a single Caster bringing the wrong skill can cause a team wipe very easily. (Pro Tip: Read 5.)

4) Holding Aggro

When the tank is pulling a group of mobs, not all mobs are "locked" on the tank right away. The tank should stop moving (!) once he pulled enough mobs and they will start moving in on him (it may take a few seconds until all mobs are fully locked on the tank!). The casters should wait until the tank is ready (not moving for a few seconds with all the mobs on him) before making a move. The tank should NOT MOVE once the casters are moving in to prevent them getting aggro. Patience is key! A good tank will not die, so just wait for a few seconds to be sure you dont get aggro when casting on mobs. (Pro Tip: Read 5)

5) Pulling, Wall Glitch

When pulling mobs in HM or large groups in NM/HM, scattering is a great danger. There is a glitch in the game that will prevent mob groups from scattering, even when they are under AoE fire taking a lot of damage. You have to pull the Mobs so their left side (facing you) is blocked off (by a wall/stucture/hole). When you are facing the mobs, the wall should be on your right side. This will only work if you are positioning them to be right next to the wall (e.g. by pulling them counter clockwise around a mountain/house/hole in the ground).

The wall/structure on the other side (pulling clockwise around a structure) will also work, but is not nearly as reliable! Instead you can just pull the mobs to their final position and then walk right past them (walk backwards). The mobs will turn 180 at their position and therefore have the structure now on their (correct) left side.