The act of "tanking" is a massively constantly healed and fully armored melee archtype (usually) that becomes and remains the target of a monster. Because he has specced defensively he is unable to deal damage and only deals massive numbers of what's known as "threat", where the higher the threat you have as a player the more inclined they're going to attack you.
Why it Sucks
To get a grasp of how this makes sense is provided by Daesu from another thread (I recommend a reading of the whole post, but this portion is quoted for relevance to this thread):
There is a schoolyard fight. One kid begins by calling another kid names. "Na na na na your mother's so fat she makes Norgu look thin!" Says Tanky McGee. Even though this a lame, it ticks off Mon Ster jr. Mon Ster is so enraged by the taunts he keeps slugging away at Tanky McGee; but can't hit him because Rayne Ger threw dirt in his eyes. Meanwhile Ela Mental punches Mon Ster jr in the back of the head. End Result: Mon Ster jr is lying on the ground because he was a fool and kept focusing on the childish your momma taunts. Not being aware of where the real danger was coming from.
Bascially the tank does nothing but make enemies hate him through high "threat" while the healers heal and the other classes kill the baddie - i.e. the important stuff.
Without a much nicer way to put it, this makes zero sense whatsoever. Without threat the tank is entirely useless. They take no damage, making them the least priority, and they can only do jack damage, making them no concern. Just like those "tanks" you run into randomly in PvP, they are a waste of a team slot.
The monsters in standard MMO's follow the exact opposite train of thought: they become made to see tanks as the most vital target. They somehow see danger in the fact that they take no damage from him and no damage to him. They COMPLETELY IGNORE the facts that the reason the tank isn't dying is because someone's healing him, that the reason the monster himself is dying is because of not the tank, and that everyone else besides the tank is wearing far less armor.
Essentially, monsters in MMO's are made to be stupid. Because of this the developers have to have the monsters deal absurd damage and have absurd health - which fails to make the game interesting because they're still just as stupid.
There are very few things ANet simply cannot do to Guild Wars 2, and this is one of them. While what we had in GW1 was far from perfection, it was solitary proof of a much more interesting, replayable, and skillfull type of PvE. There is room for so much more improvement and potential, and for ANet to completely ignore all that would make their game play just like all the other mainstream MMO's out there.
ANet has the chance to make one of the most challenging and unique combat systems ever seen in an online game. The question is: Will they take it?