What makes the game worth $15 per month?

Things are pretty hectic at work this morning so I thought I'd make a quick post and ask you, dear reader, the ultimate question. What makes WoW worth spending $15 every month for you. If you're just in need of an mmo fix there are plenty of other free options that are quality products(LOTRO, DDO etc...). What is it about WoW that has you plopping down your hard earned simoleons every month?