I'm going to get WoW, what should I expect the most of it?
Grinding. Lots and lots of grinding. 
Then you finish and you PvP. Then you die a lot and get into raiding. Then you gear up and make ANOTHER character and grind some more. 
End game goal: Get geared up to increase the size of your awesomeness. At which point, the game is reduced to piles of mods and calculations to further increase your tanking/healing/damage-dealing potential. 
Then you buy the next expansion when it comes out and do it all again.