The overworld will be dotted with multiple dungeons, some of them quest-specific and others that are random. Regardless, Blank notes that players should "always feel rewarded" plumbing the depths of any dungeon. Also, players won't have to worry about backtracking out of dungeons, as each will have a portal back to the surface at its endpoint.
Runic has also posted a lengthy level design Q&A with plenty of technical details, including information on level-editing tools. The Q&A also confirms that outdoor levels will feature a day and night cycle and random weather.
Check out some fresh Torchlight 2 screens in the gallery below.