I'd say the materia system adds a level of customization that most MMOs do not get into without costing money. The Crafting professions actually feel like important roles in the game, and not just something to complement combat, or busywork for some achievements. The community certainly, though it remains to be seen PVPs role. Finally, Limit Breaks requiring diversity in parties and providing new methods in carrying the day through teamwork and strategy instead of fillling the party with "that one class" and a healer.