The breed of “Action RPG's” spawned by the success of the Diablo series has never brought about a great deal of ambition. I suppose there is the Borderlands series, which added in the moment-to-moment gameplay hooks of first person shooters but other efforts seem even more tame. Sacred 2 added an open world (with enemy levels as barriers because devaluing the character progression would take out half the genre's appeal). Path of Exile has an extremely large skill tree to wind your way through. Torchlight II is conservative even for this context. It does nothing resembling a new wrinkle for the genre and it doesn't present the core hooks in a refreshing way. Developer Runic Games simply took that core reward loop and spaced it out just right.
It's interested that these games do get labeled as “Action RPG's” even though they hardly have any action or role playing in them. They don't resemble the action genre much as there aren't any of the skill requirements you'd expect. Holding down the mouse button automatically keeps you on your damage-per-second schedule and most builds merely require you to hit ability buttons as cool down periods wear off. The prospects for role playing are similarly meager. This particular game has some distant backstory about a bad guy in need of a serious hacking or slashing (and pretty much all these games have some similarly flimsy story motivation), but it hardly matters in the frenzy of experience points and shiny loot. What motivates the player character?
Runic Games knows that it's all about the reward loop. You defeat enemies and complete quests to gain experience points and loot drops. Once you clear out an area of enemies and quests, you move on to the next which has marginally more powerful enemies. It seems you could use more levels and better loot so the cycle continues. It's a tried and true cycle that Runic made no attempts to add on to. Instead they paced the drip of rewards just right. There's no need to stick around in one area too long as the carefully balanced enemies continually push you along to their successors. There's no need to make constant trips back to town because you have a pet that can cart off unwanted loot to sell and come back with a fresh stock of restoratives.
More than the adrenaline rush of an action game or the character drama of a role playing game, this reward loop has a relaxing effect. It's designed to constantly give positive reinforcement to playing while never really expecting much to get those results. This is behind the cleverness of the visual design of Torchlight II. The visual diversity does have the practical impact of minimizing the potential monotony inherent to the nature of simple systems played out for dozens of hours but it's the stylization that has the biggest effect. Vivid colors and cartoon proportions wash out the subject matter of blood and bust. This is a pleasant experience.
The most basic appeal of this little genre, and Torchlight II because it sticks to the formula so closely, comes down to a statistical certainty. That next level is always so appealing because the enemies always have just enough stat points to be a barrier and the ever present next level gives you a statistical certainly regardless of your skill. This hook has been around nearly as long as the medium itself and so few developers choose to focus so exclusively on it. In the same way, I can say with a good degree of statistical certainty that you'll probably want this game. There are just more satisfying efforts out there.