One week, my Coil group pug'd a WHM for turn 2 when our usual member couldn't make it. On ADS, I popped Fight or Flight, Shield Lob, Circle of Scorn on the way in, threw out an uncombo'd Halone for the hell of it... Then ADS turned around and cleaved- nay, devoured our back row. "Bwuh?" I thought, "how did I lose aggro so fast?" Then I glanced at the party list and saw the regen from Medica 2 sitting on everyone's bars. The WHM was so far ahead of me threat-wise my bar was probably 25% of his.
So yes, gonna echo the "don't regen before pull, don't use AoE heals too early or if no one but the tank is hurt" advice.The overheal aggro can be really bad, especially in an 8-man.
Aside from that, if the healer is DPSing, I would also request that he/she not wait for the tank's health to get too low before switching to heals. This didn't happen to me, but I was with a tank friend in Copperbell HM where the healer let him drop below 100 HP on trash (he was Holy spamming). That gave both me and my friend enough time to say "Ohshi-" before the healer popped Benediction and walked on like nothing had happened. I'm cool with that if I'm with friends, but if I don't know you and you let me get that low, I'm going to think either you're tunnel visioning or you hate me. ; ;