I think the one thing I like about MMO's is the fact that it's literally an endless game. On WOW, their are acheivements, dungeons/raids, PVP, and vast character selection. You can't 100% complete an MMO and said you've done 'everything'; that's just impossible.
One other reason is the person to person interaction. I could play with friends and with random people or against friends and random people. It brings a different social atmosphere.
Though I do like this game A LOT. I quit it for various reasons. The game gets very repetitive: "The Grind." In the end, I don't gain anything from it; it's literally pointless!