As promised, Jeff C., here are some thoughts about the design of Diablo III.

But first, consider Peggle. Yes, Peggle.

How do you play Peggle? You line up the cursor, and click to fire the ball. This is your means of interacting with the game state, right? Then, the ball hits the pegs, lighting up the pegs and making "happy" sounds - this is the game state reaction to your interaction. Finally, you are awarded points based on the success or failure of your shot (a tangible measure of your interaction with the game state), and the process repeats again.

The concept is relatively simple - press button, get shiny, repeat. When the ball physics are right, the sounds are synced with the ball contact, and you are given significant and/or particularly meaningful opportunities for big rewards, the process gets even more compelling. It's akin to Pavlov's experiments with dogs.

Now, replace ball-firing with combat, pegs with undead, and a scoreboard with loot. Diablo's combat is obviously more complicated than lining up a shot and taking it, but when you look at the abstract idea of what you're doing (short interaction, reward, repeat), you're looking at the same compulsion loop.

TL;DR - Do stuff + get shiny = fun.