Hell, Dante would have you know, is not homogeneous: Inferno is divided into nine circles, each one worse than the one preceding it. Equivalent experiences on this side of the grave are somewhat less heinous, but then there is heinous and there is heinous. If your feet are bare, almost all LEGO blocks will cause you pain when you tread upon them, but some of them are worse than others:
There are a few criteria to consider, the most important being natural resting position. Think about it this way: Stepping directly on an upright knife would be acutely unpleasant, but how likely would it be to find a knife on the floor in that position? More often than not, a knife on the floor will be laying flat and relatively harmless, which is why a LEGO piece like 2m cross axle w. groove didn’t make the list. It would be killer to step on upright, but that’s just not prone to happening.
The list contains 25 different blocks, and the worst of the lot tend to be the smaller ones, due to the laws of physics:
Small bricks are a theme on this list, due to the definition of pressure: force per unit area. In stepping on a LEGO, the force applied by our hypothetical stepping foot will be the same, so as the unit area gets smaller, the pressure, and therefore pain, increases.
It’s enough to make you put on your shoes.