It's like Diablo II: The Animated Series and reminds me of Record of Lodoss War for Dreamcast. A game that I own and played 8-9 yrs ago. Then again there is World of Warcraft, but Record of Lodoss War predates it.
Classes - In Torchlight you assume one of three different classes. Within each class you gain skills that allow you to fine tune your play-style through acquiring abilities from three different trees. The higher your level the more options in each of the trees become available to you. Any skill in any tree is available to you at any time, so long as you've gained the level necessary to open that skill. The classes are well balanced, though my limited experience with the ranged class led me to believe it might be a little under powered.
Loot - Very much like any other game from Diablo to World of Warcraft. Enemies, some quest-givers, and chests drop loot. Weapons and armor come in a variety of flavors from plain to epic. Gear can be enchanted and/or socketed to gain new abilities and/or powers. You can sell items in town or transmute them to create new gems.
Pet - This is probably the most significant change from Diablo found in Torchlight. You choose a cat or dog and it follows you throughout the game. Fighting along with you, storing extra loot for you, or returning to town to sell loot. You can equip your pet with spells as well as rings and amulets to increase it's ability in combat. In addition you can feed it fish to grant it certain powers for a period of time.
Etc. - One feature that really stands out is the shared chest. Each character has his/her own personal chest as well as a chest that can store items for use with any of your other characters. This is incredibly useful as you're constantly picking up unique or rare items that other classes can use. The shared adds alot of replay to the game. The graphics are great as well.
The game play is a lot similar as Diablo with few improvement:
1) Tons of storage, so looting is much easier, esp. when you can ask your pet to go back down to sell the stuff. I didn't use one single town portal in 3 hrs play.
2) Easy to save, you come back whereever you exited earlier, and pick up exactly what you left before.
3) Easy fast key, you just use 1-10, you can put anything in there from lotion to spell to attack.