For WoW, the system isn't built like that, so there'd need to be some basic changes. If everyone is getting whatever the item is (like how quest items work), then it probably doesn't matter, if it's for some bigger item, maybe a ticket system or matching drop rate against damage done (which screws healers somewhat, but it wouldn't be the most common system, and party can be weighted differerntly).
The system straight from GW2 wouldn't work for mob tagging, but that's not what most of us are saying anyway. Blizzard has redesigned the talent trees, they've reworked training for skills, they've changed some of the very basics of their game, they can figure out how shared tags work. It's more a matter of discussing whether the folks would want them to rework the system to work that way.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.