Other resolution (no scaling) in MacBook Pro retina
Images and icons look about the same because of integral scaling. But text in non-Retina apps looks clearly worse. The difference is that on a non-Retina display, text uses sub-pixel rendering. But with Retina, old apps use full-pixel anti-aliasing instead. (Those full pixels are then multiplied perfectly with integral scaling.) Not quite sure what Retina text on a Retina display uses, it's hard to eyeball.

Separately, other resolutions look pretty good. The highest is 1920x1200. The trick is they are also rendered in double-resolution (3840x2400) and then scaled down for the display (2880x1800). Because there are so many tiny pixels, the result is acceptable, but the optimal setting is definitely sharper.

So Retina apps at 1920 look pretty good, and non-Retina text at 1920 has smaller and slightly fuzzier full-pixel anti-aliasing.