I don't know exactly how it's done in GW2, but I see tons of opportunities for douchebaggery, and we all know the WoW community would take FULL advantage of every loophole in the system.
For instance, what if you and your friends are all fighting a rare and some dude comes in at the last second and stabs it once...does he get loot? I should hope not, he basically did nothing.
So maybe it only counts if you DPS'd from the beginning? Like the first 80%? Well that could still be gamed...someone could simply DPS at the beginning then sit around and watch the others kill it, then collect loot at the end.
So I guess my point is...if such a system was in place some people wouldn't work together, they would just run around tagging mobs that people are already fighting, and simply collecting loot when others finish the mobs off for them.