Also see this comment on Hacker News:

http://news.ycombinator.com/item?id=4069940

Basically a 10% chance to get a reward is more addictive than collecting ten tokens to cash in for a reward. The uncertainty and chance really brings out the dopamine.

I believe the word they use is "scientistic" for analyses like this. :)

It is the will-I-actually-get-something-for-this-effort that makes gambling more addictive than traditional gameplay.

That's why Torchlight is so much more satisfying. Once your relative frustration index has jumped the shark, instead of throwing the game in the trashbin, you can use the editor tools and cheats to keep incrementally upping the awards. In fact it's even better, you can measure your progress towards "item nirvana" as there's a known universe of items, known universe of powers, and an infinitely-personalized combination of paths to get there.