[Digits]

UO's skill and stat progression system. Fuck filling bubbles when I can progress my character in a more concrete and logical fashion.

EQ's diety, alignment and faction system. There is no better way to define your character than by choosing what faction you want to oppose or align yourself with. How many people went out of their way to be KOS or friendly with certain racial capitals?

DAoC's realm versus realm system, consisting of real time keep sieges with -three- factions vying for control of land and relics. By having three factions you have an extremely tenuous and fluctuating power struggle and make-shift alliances and backstabbing becomes a fundamental part of PVP, even though you can't say a word to any of your opponents.

WoW's ghost walk system. Say what you will about the poor penalties, but no other mmo comes close to the integration of the death system. You die, spawn in a graveyard and find yourself in a spirit world of black and white and find your way back to your earthly remains. Absolute brilliance. Why does every other mmo (that's not UO) have you spawn naked/armed at a completely random location?

EQ2's dialogue tree system. Even generic bullshit quests become interesting when you actually engage an NPC in a dialogue, although half the time the dev's don't bother putting in the necessary effort. When they do go through with it, though, it's fucking awesome.