Pete, I think it's mostly the game itself. At this point, there is definitely a general MMO culture shaped by past games that carries over into new games, but each game shapes its own player-culture to a large degree.

When I played WoW, my only experience with Blizzard was Diablo 2. I remember being greatly disappointed at the relative lack of variety in both skills and gear. The skill trees are similar to D2, but only a shadow. There's a wide variety of loot, but players have a means of achieving loot rather than discovering it: the auction house. And the auction house is just a symptom of a larger focus.

The loot system in WoW and similar MMOs is inseparable from the overall design, which focuses on gradual achievement (conducive to addiction modeling — the surest, though not the only, means of retaining subscriptions) and rigidly defined roles. Gameplay is effectively like a funnel (wide to thin)… there are more gameplay options early on and the variety of viable playstyles shrinks as you level up.

In games like Diablo 2, there is no optimal loot because gear's efficiency is situational. Different enemies are immune to different types of attacks and inflict different sorts of damage. Some enemies evade the player, rewarding ranged attacks, while others stand there and beat on you. Whether a slow-and-heavy weapon or a fast-but-weaker weapon is best depends on how many enemies you're fighting at any given moment.

Diablo 2's skills allow for so much variation within a single class that five members of a class can bear little resemblance to each other. One necromancer can focus on damage modifers, another on bone projectiles, another on elementals, another on skeletons, and there are countless combinations. A single skill can be upgraded much more than is possible in WoW. The game includes roles, but those roles are very loosely defined. Emphasis is on emergent and unique experiences, rather than a scripted avenue.

Dynamics make personal preferences and choices relevant. Typical MMOs allow for customization, but reward choices which fit the developers' static encounter and grouping plans. Diablo 2 players have enjoyed multiplayer for years without need of power evenly distributed between them, predictable difficulty, and uniform gear standards.

I think I got off on a tangent…