GW2's dye system.

Archeage's seamless world and diving (or at least the seamless diving, come on man.)

SWTOR's voice acting for every NPC in every cutscene

Star Wars Galaxies' house decorating system: All furniture pieces in the game can be placed in your house and then rotated and moved in any direction along every possible axis. As such, do away with "outdoor" and "indoor" furniture and let us finally move them up and down freely regardless of any surface to place them on. Let us rotate them to lay them on their side too.

Virtually any Korean MMO's character customization. Give us sliders to make truly unique characters.

Take a page out of City of Heroes / Champions Online for how characters look as well. Wear whatever you want. Let us use ANY clothes or armor for glamour even if we're tied to specific weapons.