This is not your father's MMORPG. It's your 8-year-old nephew's MMORPG.
Don't get me wrong-- WoW is a solid game that can keep you entertained for a good while. But it's a very very simple game, with next to no room for building a character, or exploring, or departing the rails in any way.
If you're happy being "Warrior #99382992", and essentially indistinguishable from every other player of your class and level, then you'll have a good time. If you like to occasionally jump the rails and do something new, you won't like WoW.