Perspective-correct texture mapping
Texture interpolation that accounts for depth, so patterns on tilted surfaces shrink properly toward the horizon instead of swimming.
Texture coordinates aren't linear in screen space — a row of tiles receding into the distance gets denser the further away it is. The fix is to interpolate u/w, v/w and 1/w, which are linear, across the triangle and divide back out per pixel. That costs a division per pixel, which was expensive enough that the PlayStation skipped it and settled for affine mapping.
Quake's software renderer (1996) split the difference: it did the true divide every sixteen pixels and interpolated linearly in between, with the floating-point division scheduled to overlap the integer work. The 3dfx Voodoo did it in hardware the same year, and it has since become one of those things nobody notices until it's missing.