Paint.net took over as my default image editor a long time ago. There is one very simple feature however that is missing, but is present in all other applications I've used (Photoshop, GIMP, etc.). When you're editing a part of something you're designing you often want to "flatten" it into a png and then move it into main thing you're making. However you have to flatten it, save it as that png file, then hit undo to put it back into a .pdn format in case you want to edit it in the future. A Save as Copy feature usually solves that problem without the extra annoyance and easily saves you from accidentally destroying some hard work.
