So far I haven't been involved in any raids during live, the few I participated in during some of the testing a while back weren't too hard and it didn't seem like heling was much of an issue.
Having said that, I've been a raid healer in other mainstream MMO's. In those the best strategies for main tank (MT) healing were direct heals as needed and a heal over time (HoT) pretty much maintained, I've barely played any healers in EnB, are there any HoTs? Spamming heals (not counting a heal chain as spamming as it spreads out the heals between multiple healers and is controlled and by it's nature an aggro management tool) has always grabbed agro from the mobs especially any group heal, also most other threat/hate/agro calculators not only counted the actual amount healed but also any overheal that occured if the healee wasn't low enough HP to need the full heal.
Now, like Kyp said there needs to be some balancing of mob damage to ensure that every healer in range isn't having to spam their largest heal but has there been any experimentation in heal styles? If so, what kind? From what I've read here it seems like most healers are using their highest level heals all the time but maybe I'm getting a wrong impression. I know as a past hardcore raider that figuring out what tactics to use in certain situations is part of the fun of the whole thing, maybe we just haven't found the optimal way to heal yet.
Do the deaggro abilities and taunts (are there any forced taunts?) work at lower level? In most other MMO's aggro managment abilities are downright unresistable by any mobs, especially raid mobs since that is where those abilities are actually needed, or if they aren't completely unresistable they have a better chance of landing then not by an appropriately leveled and geard player.
Anyways, just my two cents on the topic, really looking forward to seeing how it turns out.