I snagged this brilliant comment from Slashdot:

(source)
They developped as much content they could afford, and messed with how much they give you at each level. If you plotted a graph with the time along X and the percent of content you’ve seen as Y, let’s just say it would look like very much an asymptote. It starts by going up pretty quickly, but then it slows down, and it takes more and more time to get closer to that covetted 100% spot. By the end of it, huge amounts of time are required to make even the tiniest of progress.

I fondly call it the “boiling a frog alive model“. They say that if you put a frog in hot water, it will just jump out. But if you put it in cool water and slowly heat it up, it will stay in and get boiled alive. Now I don’t know if that’s true with frogs, but it’s certainly true with about half the WoW players. Because that’s what Blizzard does.