In WoW there used to be a design philosophy that dungeons were part of the world, even though they were instanced. So even though you didn’t camp bosses or random PVP in them as you might in Everquests non-instanced dungeons, there were reasons to go inside other than the loot the bosses could drop.
Last night I helped out my partner by sneaking into the Zul Aman heroic 5 man dungeon on my druid (with his rogue). He had been sent a load of the Amani Hex Sticks, dropped by mobs in that raid which are used, primarily, to hunt for a rare companion pet frog.
The sticks are used on normal frogs near a specific pond in the instance, each frog is transformed into a random NPC (you free them from a witch doctor’s hexx). The NPC might give a little story before disappearing, might turn out to be a vendor selling a small set of unique items or if you are very lucky you end up with mojo the pet.
This kind of unrelated activity, within a dungeon is or at least was very common in WoW. There were special mount quests, rare crafting recipes that spawned on shelves in corners, rare materials to gather or even seasonal NPCs to visit for a festivities quests. They help tie the open world to closed world of dungeons, and give you reasons to return to older content beyond just farming items for twink alts.
I believe that Blizzard moved away from this as they simplified the game, especially with the Wrath and Cataclysm. I haven’t really encountered this much in other games actually, though I haven’t set foot in many dungeons in EQ2 the most likely candidate otherwise for alternative use of dungeons. Are there other examples?
Certainly the more modern games, like Rift and SWTOR (and I presume Tera) are pretty narrowly focused on dungeons being solely there as group-oriented loot and XP piñatas. I can’t help but think this is a pretty big reason why I haven’t stayed interested in them in the longer term because they’re so one-dimensional.