Given the experiences I've had in 5-mans and certain group events, Dorfadin has a point. I know Meledelion will disagree :P, but aggro in GW2 has always seemed really chaotic and I don't see why that'd change for a "raid".
I actually think in some respects it would be better if encounters were designed similarly to WoW at least in terms of knowing what to expect for certain bosses. Maybe they always target the farthest player, or that they do massive AoEs if people are too clustered together, etc.
Maybe I'm just not open-minded enough for GW2's group combat system. I can't help but sit there and ask WTF did I do to get so much aggro that I'm literally tanking in downed status? Why is it I go into a fight with a champion at 50% HP, hit it once, and then have to run out of its range to get it to leave me alone after that? I don't like perpetually feeling like Red Riding Hood in Karazhan. :P