Diablo III: Pre-ordained skill trees. | Torchlight II: You choose everything.
In Torchlight II, you'll have far more control over your character build. I've been playing as an engineer, and have been choosing from among three different skill trees, each of which are tied to a different kind of combat—two-handed, sword and shield, or gadget-based. It feels much more like a standard RPG (or more like Diablo II) than the slot-based, interchangeable upgrades of Diablo III.
On a related note, it's also worth mentioning that Torchlight II's skill trees are much more permanent—you can undo your last three skill upgrades in town (for a price), but you can't just swap your skills around all willy-nilly like you can in Diablo III. It's more restrictive, but also truer to its roots. It could be that you can fully re-spec on New Game + or something; I'm not that far yet. It'd be nice! But when it comes down to it: Not counting the mouse, Diablo III has four hotkeys for powers; Torchlight II has ten.
I decided to ask Max Schaefer, Runic Games' CEO, if the lower price and smaller budget of Torchlight II inevitably means it will be seen as a cheap-and-cheerful version of Diablo III. He says, “Back in the day, when we would sell a $60-dollar box, by the time the money came back to us, it would be maybe $14 dollars a copy. And that's what we get nowadays on Steam, selling a digital download version for $20. So from our perspective, we're as viable and as profitable as an independent developer used to be selling $60 boxes.
“We're bypassing the publisher and box distribution and that lets us bring a $60 game to people for $20. We don't want people to look at Torchlight as a cheap game, because we think it's competitive with anything out there. But we like the idea that we're at this price. It sets us apart a little bit. We also want people to buy a Runic Games product without even thinking about it, because they know it's always a good value.”