Essential feature?
Don't be ridiculous.
Tens of thousands of people play OpenXcom without ever noticing anything, including myself.
That's because everybody forgot what the game was supposed to look like, and this thread is a testament to that. Because NTVDM and DosBOX (unless you knew you should set it up for non-square pixels) had the same problem. Stuff like that is easy not to notice. Hell, if Geoscape was flat in OXC, everybody would get used to it by now.
Anyway, I've miscalculated in my last post, 1:1.2 pixel ratio is actually correct. So some unevenness and/or blurring are inevitable. But this hardly matters on higher resolutions. 4K monitors are going to become common in a few years. In fact the blurring should be barely noiticable on 1440p monitors.
The proportions do look a lot better when you use non-square pixels. What I have to ask is what engineer thought it was a good idea to make pixels not be square?!?
This resolution was originally intended for 40x25 text mode with 8x8 letters. Letter resolution was limited by byte size, and stretching square letters vertically was a reasonable way to maintain their proper shape.