I like both, instanced for when the devs want to challenge a party and open world for most everything else. There is no reason we shouldn't be able to have lots of both types of content.

There should be rare spawn open world NM's for those who want to do that. I'm even ok with a system (pop-items or timers or such) making it so people have to take turns for claim on some of them. However there should be the whole range of opportunities from the cutthroat competitive stuff to 4 buddies enter an instance and have the place to themselves.

If the game is too heavily instanced then it will no longer be a proper MMO in my mind.