Would it be correct to say (as regards other MMOs):
1. When fighting a group of mobs there aren't a lot of weak ones.
2. The other mobs take a long time to kill.
Not necessarily, but the tank will normally be able to aggro the whole group, so it's often a case of focus on the big guy, but use AoE skills which take down the weaker ones without the bother of targetting them.
3. If the dps tried attacking the other mobs, they would get overwhelmed and would die.
The DPS can often be weaker than the tank. In addition where there is range and melee DPS, it can cause the mobs to be running amok all over the place. This then makes it hard or impossible for the main tank to regain control of the group, puts additional strain on the healers, who must now heal multple targets and is far more likely to end in total disaster.
4. It's easy for tanks to hold aggro on the entire group.
With skills such as wither/slow time and discharge/force breach, I find assassin tanking to be most like what I am used to in other games. AoE aggro building skills that allow me to control all or most of a group.
[quote[5. So killing the one the tank is attacking will ensure that only the tank takes damage during the fight?[/QUOTE]
That's the idea and allows the healer to focus pretty much exclusively. Positioning for DPS is also important, attacking from behind to avoid frontal AoE and similar.