I just levelled my second 70, after playing since closed beta. I love the first 40 levels or so. It's always a huge achievement and feel-good-feeling to get your first mount (now level 30 :x) and be able to wear the next level of gear (for shamans hunters warriors and paladins) and it really is the defining level for your class. You get access to your first 31 point talent, you unlock lots of new abilities, and for most classes, some sort of quest line. But between level 40-60 its all pretty samey. Of course, after 60, since the entire game was revamped for Outlands, it really does revitalize the spirit of the situation but even that dies down after a bit.
But like most MMORPGs, the game is meant to be played at the end-game. Raiding, competitive PvP, and grinding, are all the core elements of WoW and are exactly what A) keep people addicted and B) turn people away.
If you have ever browsed the WoW Official Forums, its full of people "Zomg I hate this I will never play wow again" yet they are posting on an ACTIVE account that is paying for a subscription and they RARELY ever cancel.