A central reward in most MMOGs is leveling up. You play for a few hours and then ding! you've gained a level. And a common design feature in most games is that the higher your level, the more time it will take to get to the next level.
The PlayOn folks have a nice graph of time to level in World of Warcraft as a function of level. I redrew it as level as a function of time. This view highlights the fact that your reward frequency diminishes over time. Early on it takes 2 hours of play to level up. But the rewards come more and more slowly to where you're playing 10 hours per "ding!".
It's weird that rewards in the game so predictably slow down. The rewards themselves don't get more valuable; going from level 44 to 45 doesn't really get you more than going from 4 to 5.
Maybe the time spent makes the rewards just seem more valuable? Level 60 feels like a big accomplishment given the time involved. But then, that's totally artificial. Or maybe the game developers cynically know they can get away with less frequent rewards because there's so much sunk cost. Players spend so much time getting to 44 that they'll spend longer for 45 because they're already invested.