For me, this is "MMORPG - the thread". It all started with that accursed game called "Phantasy Star Online". Great atmosphere, awesome music, spiffy SciFi theme and neat monster design. The game itself, eeeeeh. I mean I enjoyed collecting loot and whatnot as much as anyone, but I'd be lying if I stated that I'd play the same game (mechanically) with a different skin and OST just as fervently... Ok, not a good example. I'd play the shit out of it. Dammit. Still, I wouldn't be enjoying it half as much.
I spent countless hours in the WOW starting areas. Night elves and blood elves in particular. The bloom and the atmosphere and the music and the artwork and ...
I think my highest levelled character is Level 50-something. I lost much of my interest in the actual game loop after around Level 20-ish. Just kept on trucking to experience the areas and scripted events until I got fed up with getting ganked by the other faction or got oneshotted by mobs.
I bought Guildwars with all expansions and Guildwars 2 just so I could walk around and bathe in the OST and artwork. Sure, I played quite a bit as well, but honestly, appreciation of the countless manhours invested in creating those worlds has to account for at least 50% of my motivation. Even desolate places are still so beautiful.
Heck, I even bought the FF14 CE after having played the abysmal beta/stresstest just so I could roll around in Final Fantasyisms for a while. I even bought into one of the later "all in one" FF11 packs to check out the look and feel of that world. Didn't stick around, but had fun with the free included month or whatever.
I have also spent way too much time idling in the starter area of Ryzom just to watch the weather/season cycle and the scripted Enemy NPCs migrating around accordingly and preying on each other etc.
Rift as well, although the invasions are admittedly an active part of the game I never could withstand the urge even to join even the low level ones just because I felt like beating back an invading horde, not necessarily because it was fun or for the loot.
...also, Metroid Prime. All kicked off due to that shitty landing and the rain and the music and dammit where is my Wii I wanna play...