I think it is both fault, and it depends the cases.
Every time I overgear the tank:
When I'm dps, I can pull off aggro every time I feel like to do it. Just put all my cd on a single mob.
When I'm heal, I can pull off aggro every time I feel like to do it. Overheal is my second weapon.
Every time the Tank is exp and geared:
When I'm dps, I can make an aggro war with tank and he feels it, and he let me eat my mob when he needs do other things.
When I'm heal, I just regen tank a millisecond before the pull and the job is done.
Most of the blame imho goes to dps, because dps can stop when aggro warning lights on, but he doesn't care. Insufficient tank may slow down dps for this, but dps can avoid get aggro in every situation.
Different story for the healer, a legit healer that does not want get aggro on purpose needs heal anyway, so he takes aggro if tank does not do its job of at least a minimal aggro on all mobs involved.
Last but not least, the teamwork of raiding and PF. Everyone needs do its job correctly, if hate goes wild someone done it poorly. Cannot count all the times I had as a tank to move a mob somewhere and it comes the idiotic dps that pulls off aggro when I'm moving it instead of dps other mobs he had to or dps him when it is the correct time to do it.
Want make a duty the hardest of all the game? Just make one where dps must hold the trigger for one second or do other things than dps. Steps of Faith anyone?