Source: Steam Community (opens in a new tab)
Changes:
The game over screen has been re-designed.
Made significant changes to the first dungeon and almost every hidden item room in the game.
Added medical boxes. These have replaced the wall mounted medical cabinets.
Added ammo boxes that refill all of your secondary weapons' ammo. These are somewhat rare.
Added destructible crates that have a chance to drop health, ammo, and money.
It is intentional that the second player can use the second shotgun too. The description has been updated to reflect this.
Adjusted the damage scaling of multiple shotguns slightly. The new damage totals are: 1=100%, 2=150%, 3=175%, and 4=200%
Attack tiles are somewhat more predictable now. They shoot rocks in 8 directions with some variance rather than spraying them in completely random directions.
Books can now contain graphics other than just text.
Lesser items that drop over obstacles now appear nearby the obstacle instead of floating towards you.
Players can no longer pick up or buy items they don't need.
Made cracks in blocks and walls much more difficult to see. Can't have everyone finding everything now can we?
Bug Fixes:
Selecting continue at the title screen repeatedly would eventually result in a crash.
Using a book would sometimes cause the second player to become stuck.
The second player did not take damage from poison. However, the first player took double damage from poison if the second player was present.
Updates sometimes caused you to get stuck in the wall when continuing from a previous version.