Instanced:
Strategy-based dungeons (this is not including things like the current Cassiopeia Hallow that is just a zone imo)
Most Event related content (storylines, seasonal events, pvp)

Open world:
Zones (over worlds, under worlds [not to be confused with dungeons, these would be like "sub-zones"])
NMs (yes I do mean out-claiming other people, it was one of the most excited and heart racing things to do in other MMOs)
Some Event related content (seasonal events, pvp)
Open world Dungeon (there is no big boss at the end, its just a zone with treasure chest like the new lv15 dungeon in XIV and the strongholds)

That is how I feel it should be divided, and currently in XIV they are right on the money with it. I really hope they don't lose sight of that because its something I feel has been implemented perfectly.