Aggro is based on lots of things from what I can tell -
Damage, certain abilities, proximity,and the one that seems to make a huge difference is ressing - stopping to res someone in large scale events seems to virtually ensure you're targetted - obviously that's good AI from the mob's point of view but it can be quite nasty if you don't keep your finger on dodge while working a res.
And yes, there's no full 'tank' class in game - heavy armour guys with a toughness and vitality build can maybe take 3-4 hits instead of 1-2 in my (albeit limited) experience.
Thanks for that, looks like I might need to get some toughness in my glass cannon build
