Much too early to conjecture. I'm sure the dev team knows about the issues involved and I think the majority of the players would adapt to whatever system is in place.

But I understand instancing is usually only about 1 group per instance. Where do you get the idea that multiple parties will be allowed in massive dungeons? They said sought after items will be instanced so I don't think there will be much drama in the open world on who gets the aldgoat horn...

If they allow multiple parties to engage and "help" defeat an NM, I think most people would welcome it if each party gets their own loot table. I don't see why only one rare item should be awarded to the first party that claimed or the party that does the most damage. Why not, for example, give each party that participates a certain amount, say at least 20-30% of damage, a chance of getting some rare loot, and those who merely participates can get a chance at common loot. Each winning party can each get their own Ura skin for example. But most likely, everyone will get nothing cuz the drop rate for it is so crappy...~.~