In my opinion, in almost all premade videos i have watched in last years there are always two mistakes:
1.- Ppl dont play close enough, moving someting like a ball or birds, using walls or other element to protec sometimes against ranged toons and force them come to melee, in case the group cant not charge them, etc. The only organization is just focus this, focus that.
2.- That a tank dont be the main focus dont mean can run free full time. He only has one anti ccs every 2 min, the rest of time must be snare, feared, stunned, knockback, etc. Ppl just think kill the focus and forget support rest of team. The idea of a tank is not focus, then doesnt exist, makes they are top killers.