Torchlight II will go above ground to the continent of Vilderan--though "deep ominous caverns" are still promised--"with weather, time of day cycles, and random events."
More importantly, online and LAN multiplayer arrives complete with a "free peer-to-peer matchmaking service to meet new players of a similar level and interest."
Modding is bolstered by an updated version of the Torchlight editor 'TorchEd' as well as support for playing mods cooperatively. Other exciting new things include "all new character classes" with "cosmetic" and "gender" choices.
Torchlight II will be released "initially as a digital download" in Spring 2011 on unannounced platforms. For reference, Torchlight was released for PC and Mac.
Runic is also working on a free-to-play Torchlight MMO.