A diabolical sin to any action RPG gamer!
A very crucial element for its addictiveness is the innovative save system. It does not allow you to save as you want like the original Diablo, rather, it has waypoints you can access later via teleportation that serve to save your game too, but you will be ousted out to the main menu after which and all non-boss monsters will be re-spawned whenever you load back your saves. This is the damn reason why it's so addictive. You want to keep moving on to the next waypoint to avoid all these enemies that you have fought before (though they are good level-up target practice), and the combat system is really fantastic such that you won't feel bored with it anytime.
All characters have distinct skill trees that allow several combinations which translates to tons of replay value. Other than leveling-up, the other addictive attribute is the element of treasure hunting, ranging from unique rings to full armor sets, so even if you reach level 99, my bet is you still wanna play on to get that very rare item to complete your Sorceress set.
KingLim