Bumping this thread. I think it's very important feature.
Let me give some examples from practice. I'm making animation in PaintNet, and I have about 241 layers representing individual frames + about 4-6 helper layers + background. I save frames in PNG by hiding all other layers except needed frame + helper layers. Now, you can imagine how tedious it is to save individual frame:
Make needed frame visible
Save As...
Select .png type
Press save
Press "flatten layers"
Wait while flattening...
Press Ctrl+Z
Wait while unflattering...
Scroll 200+ layers up to find visible layer (after unflattering layer view is reset to bottom layers)
Repeat 250 times...
I guess, it's easy to see how simpler and faster it would be with "Export PNG" feature. I don't think its principal design issue. It's just minor additional feature.
As a programmer, I fail to see that it is extremely hard to implement. Just leave "Save as..." as it is, it's fine. It's only a matter of adding additional File -> Export -> PNG menu and firing up same algorithm you use for flattering PDN file in background, without affecting current opened document state in any way.
Anyways, I think PaintNet is best 2D graphics editor ever, its just that this small lacking features making me really sad. But I'm still using it, coz I love it!
Thanks for attention.
P.S.
Another useful feature would be ability to save all layers as separate PNG files, but that would be a bit more complex to implement, I guess.