I will echo many other reviews and start by saying: If you liked Diablo 2, you should really enjoy Torchlight 2.
In fact, at times I felt this game was almost too derivative. Both games have 4 Acts. Each Act has its own hub town, bosses, unique geographic setting and a narrated cinematic at each Act's beginning. Even the storyline has a lot of similarities.
With all that said, Torchlight 2 clearly wasn't designed to be novel. Instead they set out to do the dungeon crawler genre really, really well. And they succeeded.
The gameplay is fast paced, addictive, loot filled fun. Everything about this game is user friendly. It's easy to get to and from town; your pet can easily be sent to sell off your unwanted spoils while you keep adventuring; even playing on the Veteran difficulty I almost never had to buy potions or magic scrolls; quests are clearly marked on the map, never leaving you wondering what to do next; when you die you have the option to respawn in town for free or lose gold but respawn near where you died.
My main criticism is likely to be experienced differently by each person who plays Torchlight 2. I started to get bored with the game about 3/4ths through the story mode. This is common for me with games of this type. I should note that I played to the end of the game fully in single player. Your mileage may vary particularly if you plan to play online with friends.
I'll close by saying if you did NOT like Diablo 2, or other highly rated dungeon crawlers, do not be swayed by all the positive reviews this game has gotten. Trust your gut; I suspect that the type of person that will enjoy this game already knows that they will enjoy it.