My biggest issue with non-tanking classes would be the lack of an actual taunt which is kind of the whole idea behind the tank. Absorb the damage and keep aggro off the rest of the party. I guess if you were putting out sick amounts of dps on all mobs, you would be able to maintain aggro that way, perhaps, but I think eventually that strategy isn't going to work as well as we might like/think. Just how exactly is a non-tanking class going to continually grab aggro from other party members? This becomes even more important since there isn't an aggro indicator like in most traditional MMO's.
I don't have any experience with tanking in the game at this stage. I'm not sure how taunt skills work when it comes to pvp either. I would assume they force other players to target the tank for 'x' amount of time. If you can't force the focus of attacks on yourself as the tank, how is that going to work?
Or are we basically talking about a shift in traditional tanking equaling a shift in traditional tactics, for example, dps knowing there is no taunt to save them so they have to go easy on the attacks so as to not draw unnecessary aggro? Seems like that would be considerably more viable PVE rather than PVP. Or is it redefining 'tank' as one who can take the most punishment and still stand, versus the taunting and forced aggro control?