If you think about how levels were structured in Torchlight, you'd realize they were pretty straightforward for the most part. Sure, there were a few detours here and there but they didn't deviate far from the path you're supposed to take or served as an alternate route without much difference. Torchlight 2 allows players plenty more freedom to explore areas thanks to its massive overworld areas and new path types.
For those not in the know, Torchlight's levels were designed in "chunks" and arranged in all sorts of manners to make them look different every time you visited them. The largest levels in Torchlight are comprised of six or seven chunks in size but Torchlight 2's overworld areas completely trump them by being anywhere from 18 to 23 chunks big.
This diagram offers a few sample layouts of the overworld areas and how they compare to Torchlight's dungeons:
Everyone is encouraged to search thoroughly through the overworlds to encounter a random event, discover a landmark, or maybe find a random dungeon to score some loot.
As for dungeons, three-way junctions are being introduced for more exploration opportunities and end caps better define dead ends. If you do decide to go off the main route, you're guaranteed to find a bit of treasure and a portal to zip you back to the dungeon entrance.
One other level type being introduced in Torchlight 2 are passes, a transitional course to get players from one zone to another. Passes are randomized and feature their own themed appearances to stand out from other areas.
Modders will be happy to know TorchED 2 has some neat improvements included, such as a new paint tool that allows you to easily place multiple instances of an object without having to copy and paste repeatedly. Want to reuse a level chunk without the hassle of duplicating it and setting up props for each duplicate? Just apply multiple themes to that chunk and Torchlight 2 will know which props, actors, and enemies to spawn when that chunk appears in certain dungeons.