As I'm listening to the PAX 2011 panel, something clicked in my head while the lack of Tank/Healer/DPS roles was being discussed. The difficulty and skill cap of GW2 PvE content will be very high. The speaker they had on gave the example of a warrior being able to run in and throw up his shield for only a few seconds while his allies healed themselves. The warrior would then have to immediately step off and let someone else tank the damage, or risk death. It sounds like players will not be able to simply stand there and take damage the entire fight. Very short lasting mitigation skills with cooldowns are necessary in order for someone to tank efficiently.
Looking at the wiki, the skill that the Warrior used in that example is Shield Stance, which blocks incoming melee/projectile attacks for a short time on a 25 second cooldown.