My favourite feature, and I suspect of many others given how often I saw this at demo parties etc.:
The split screen, pixel perfect zoom.
I've been looking for paint apps for Android for years, and one of my pet hates is that finding one that didn't do bilinear filtering was incredibly hard (I finally found one I like reasonably well called "Artflow").
But even without filtering, having a quick and easy (one keypress) zoom that keeps the un-zoomed image visible on one side, and shows the zoomed region on the other, was vital to me, because it was one of the most frequent operations. Pretty much every paint app I see either complicates this with multiple separate views in separate windows, or by showing only a zoomed in view.
I find one of the biggest problem with modern paint applications is that they crowd the screen and don't surface the most important paint tools very well - they try to be too many things.
When I used Deluxe Paint, I wanted as little clutter as possible. I'd paint with everything off except when I wanted to change a tool, or change colour - and this to was one keypress away.
Though (relatively) low-res pixel art also tends to lend itself to using far fewer tools. I tended to mostly use the "spray can", smooth, and zoom in and draw individual pixels.