The other day I was considering dropping back into WoW after an absence. I can't remember now why I've been away, though I do know why I'm thinking of coming back: the ding.
You know, that lovely, orgasm-like lightshow that tells you when you've gained a new level. Sure, it's fun to whack things with a sword, claws, or bolt of eldritch energy. It's a joy to spend time crafting various engineering toys. Hell, even fishing's a pleasant way to spend a little in-world time. But let's face it: at the end of the day, the real reason you play WoW -- or any similar leveling-based system -- is for the thrill of that culmination of experience points. I mean, you could go in for the sheer enjoyment of chatting with your guildies, or playing around with seasonal content, but there's a reason we level up in the first place. It's such a visceral response, and a quantifiable measure of progress that I'm sure most of us wish we could experience in our daily lives.
And that's at the heart of it, I think.