Windowed mode is kind of broken atm, i'll deal with it eventually, but it's a know issue, thanks for the report anyway!
On the difficulties, well i get that usually games do that what you said, but at the same time those things usually make the game easier, like you can acquire higher quality loot and just deal with the increased difficulty rather soon, same with the xp bonus. So i've came up with this current system where enemies are much tougher with each difficulty and the xp bonus (you actually get 25% per difficulty) is less that the HP bonus the mobs get, to ensure it's not really worth it just for the bonus unless you can kill them almost as fast as you do in the previous difficulty.
The item quality doesn't increase either, but the spawn rate of champion mobs do, and since every champion grants you at least 1 item drop, you can think of it as a slight increase of drop rate.