One other thing concerning taunt. You have to be careful, because until level 60, I do not believe you are able to sucessfully taunt any mob that is higher level than you are. So taunt is not a viable option for many pop leveling parties.
I play a warrior, and I experienced an increased difficulty with maintaining aggro at level 55. Don't be too worried at 56 you get your next provoke type discipline and I found that this helped greatly.
Also concerning aggro, Critical spells and critical hits in combat do not effect aggro. In fact a miss generates just as much aggro as a hit. There are several examples of this that can be found on other message boards. The one I will use is that many guilds(including mine) when we kill seru, the main tank typically does not use a seru bane weapon (he does no damage) but does a fine job of holding aggro