@OP:

Here's what will make me quit. If by "instanced" it is always a group of people teleporting to a dungeon for ALL endgame content, then yes.
It's nice to see other people on the field and it makes it feel like a world.

RIFT is a good example of some of the things they could do to not make it raid-only (WoW-style). They honestly need to have many different variations of endgame content. If it's open world that doesn't mean people have to compete with each other. It was mostly bad in FF11 because of long respawns, low drop rates and only 1 single occurrence of that monster.