Not quite what I was going for.
I'll edit maps like this one, where I may need to go through and change the borders or coasts of a country piece by piece. While the hotkeys 'o' and 'p' don't really take very long to switch back and forth from, it is still a few seconds longer than what is achievable in MSPaint by simply using the pencil tool and using shift to draw a short straight line.
Additionally, when you use the line tool in Paint.NET it is difficult to draw a second line in near proximity to the first, as the line remains selected, and trying to click that close often simply manipulates the length of the line, unless I clock outside the proximity of the first line. This often creates a small segment somewhere else, requiring me to undo that move. Again, it seems like a tiny problem, but when I have to edit every coast's or every border's pixel it can save me an hour.