If you liked Diablo 2 and Fate, then Torchlight will be the sequel of your choice, meant for both Hardcore and Casual
Difficulty is quite scalable, you can jump in a jump easily and the learning curve is a "easy to learn hard to master" type.
The classes are quite distinct and play differently, however they are meant to be hybrid-able easily. For instance I have a Alchemist (mage) with great magic skills, but decent melee (269 DPS) at level 13. The classes feel like the most played Diablo 2 builds except for the fact that builds are much more looser in this game.
Story is great it ties the infinite dungeon, the side quests, and a main quest in well. There's also alot of suprises along the way.
The UI is quite clean and was meant to be simple, a little too simple for my tastes but works well all the same.
Lastly they have a modding tool that is simple to use and easy to modify the game in, thus the 20 dollar deal for this game is outstanding