not an expert..but you dont need one to tell you that
1. warrior are not the best aoe tanks
2. its not your fault (alteast always) when dps dies
For me the "kill the skull first" rule is everything..if someone decides that it will be nice to nuke another mob..well..."you aggro it...you tank it"
after all dps ppl can use the dps meter..why not the threat meters
u can always add some spikes on the shield to increase aoe threat