Reading the Steam page:
Free roam, non-linear, errand-based gameplay: Go do your daily chores or just screw around as you aimlessly run amok in the town!
This sounds good since I didn't really like the linearity of Apocalypse Weekend. The free rampaging was the best part of Postal 2.
I enjoyed AW quite a bit (zombies, pigeon mission, great new weapons, etc.) but having the CHOICE to kill or not in the original Postal 2 was what made it unique.