It is the rounded edges - specifically a sort of ball-bearing grind that puts a half-circle curved profile on the edges of the blade.

Part of the reason scrap yard knives are so inexpensive compared to Busse and Swamp Rat knives is the simplicity of their manufacture. Grinds are usually full flat or saber, instead of convex, and there aren't a lot of extraneous "fit and finish" operations, which take more man-hours and cost more money. Nuclear meltdown would definitely be one of those extraneous operations, IMHO.