I didn't get a chance to test pvp for the brief window it was supposedly open for, but I can imagine what problems there might be. Now I know there will be seperate abilities specifically for pvp, but the main problem I foresee happening right now is the usefulness of tanks. I was a heavy "pvp'er" in SWTOR and a big fan of their tanking system. The tanks there allowed you to place a guard on a person to divert 50% of their dmg to yourself. This way the tank could negate the focus fire of the opposing team well enough for the healers to save the player getting hit. Your team would be essentially useless without a tank in a scenario as such.
In this game however, the closest thing you have to something like that is the PLD's cover, but that lasts for 12 secs and is on a 2 min cd. Unless they implement some cool new skills for tanking, I do not foresee many serious teams running tanks.
Any other thoughts