Torchlight isn't great or innovative. In many ways, both Torchlight and its sequel are slimmed-down versions of Diablo 2. The first Torchlight really jumped out and said "Hey, I'm a Diablo clone!" It didn't really try to be anything but that. It's not like Path of Exile (which also tries to be a lot like D2) where it revamps how characters work, but keeps the Diablo-like mechanics and gameplay. Torchlight has characters and classes that feel a lot like Diablo characters in a plot that even feels very Diablo-esque.
But what Torchlight and its sequel did is really something. Torchlight added in tons of convenience features that just made the game a lot better. The new pet feature acts like the followers from Diablo 2, but is completely customizable and can cart your loot back to town to sell. It can even buy consumables from the shops in TL2. This is so simple and brilliant I don't understand why more people don't jump on it. It also adds customizable ability slots that let you tailor your strategy to fit your style even more. I don't like the way they were handled in TL (having spells that healed was an excuse to not have heals in the base classes) but they were much improved in TL2.
Torchlight as a series also said, "We want you to play the game your way." There's no forced online. Even in online play you can cheat your own gear since saves are stored locally. Rather than worry about stuff like that, TL2 just let its players play the way they want. That means that TL/TL2 can be played forever with your friends, without having to worry about whether Runic will stop hosting game servers or something.
The biggest thing that other games should take away from TL2 are the conveniences. A customizable pet/follower to sell junk, more NPCs to offer useful services, and better ease of access in general. A shared stash to store loot with a lot of space. TL2 didn't make any waves, but it did make some really useful features that other games can steal. I don't know why more of them don't copy Torchlight's magnificently useful pet.
Also, why are followers in D3 disabled when you play multiplayer? Whose idea was it to make the game much harder and take away your follower?