Having done may of the dungeons as both a tank and a healer, I can say that abilities have 'something' to do with aggro (obviously) but not everything. Hell, when I was standing in front of the dungeon the other day, waiting on another player, one of the party members was attacking a group of enemies and two of them ran to me and starting beating on me and i was literally standing there doing nothing.. and had done nothing.

The way things work in this game break the traditions we are used to and I think that there is going to be an adjustment phase (and probably not a pretty one) as tanks, healers and the DPS figure out how they are all supposed to fit into their group roles with this 'rule set'.

As a healer, I try to ask everyone to be close and not stray to far. It seems almost instinct for players to want to run away from what's hitting them, but often do it without considering what party members they are also running from. Simply asking them to be aware of staying close helps me out immensely when I'm healing as a large portion of the heals are AoE's or multi-target heals.

As a tank, I have to accept that I can't and won't have aggro on everything. Even if I could and did, I would run out of Stamina from blocking so fast, I would drop like a rock. Some of the mobs are going to run rampant and hit things other than the tank.. and then they may switch to the healer, and then decide they really like the tank. I've found the tank's responsibility is more to control enemies and keep the attention of the most dangerous ones.

As DPS, their survivability is (not unlike other MMOs) very much their responsibility. AoE damage happens and that will need to be healed, but standing in fire is.. bad; and I keep running into healing DPS that like to do that. Also it's suggested (at least from this player) to have some sort of defensive ability on your bar as DPS, or at the very least, a heal.

The roles in this game aren't quite as cut and dry as other MMOs. To be the most successful, you're going to have to think outside the confines of your own role and try to realize how you can make the roles of the other players easier.