Despite Torchlight 2 being my second most favourite ARPG ever (after Diablo 3, but before Diablo 2), I always ignored the first Torchlight, considering it inferior.
Having tried it on both PC and Xbox 360 now, I feel like I was right and wrong at the same time. It is more like Torchlight 2 is actually Torchlight 1.5.
Structure first. First Diablo game has a vertical structure. We descent deeper and deeper into the dungeon, but each level is pretty limited. Diablo 2 has a horizontal structure. The areas are wast, but if you ever find a dungeon, it rarely goes deeper than two levels. Every few levels in Diablo 1 dungeon have a common theme. And entire areas in Diablo 2 have themes as well: grasslands, desert, and so on. Torchlight 2 clearly follows Diablo 2, with even the same themes for the episodes. While Torchlight 1 is Diablo 1, where there is a single town, and you descend deeper into the mines.
What neither Diablo 1 nor Diablo 2 had from the start, though, is mana regeneration. Given the fact that skills are one of the fun parts of any ARPG, having mana regeneration is good, since you spend more time doing fun stuff.
There are three skill trees for each character, like Diablo 2. But those aren’t exactly “trees”, as you don’t have skill prerequisites. Something modern gamers criticize Diablo 2 for.