Torchlight is Diablo. With a dog.
Now, Diablo was a great game, so there's nothing wrong with that. And the dog is pretty crazy. He can carry loot back to town and sell it for you, making him more useful than 99% of all videogame AI henchmen.
I'm not really sure what else to say. Torchlight is fun and polished, with nice cartoon-ish graphics. There are three classes, and each class has three trees, allowing each class to be played in a different manner. For example, the Inquisitor can be ranged weapons, assassin-style melee, or traps. The trees aren't formal trees, as you don't have to put points in lower tiers to get higher talents. You just need to be the correct level and you can cherry-pick talents as you wish.
There are some nice updates, like a stash you can share between characters. Recovering from death is interesting, as you can either: pay xp/fame and respawn at point of death; pay 10% of your gold and respawn at the start of the level; or pay nothing and respawn back in town. Finally, there's a second track other than experience. Defeating named enemies earns you Fame. As your Fame level rises you get additional talent points. That is a nice addition.
Also note that Torchlight is single-player only, no multiplayer.
But it's pretty much Diablo with some tweaks. If you liked Diablo, you'll like Torchlight. If you didn't like Diablo, you probably won't like Torchlight.