I've heard numerous times in Tek Syndicate's videos that Japanese capacitors are supposedly "the best" and that they provide nice, "clean" power. I'm curious to know what exactly makes electrical components "good" and power "clean"; is it the efficiency? Length of life of the components?
Clean power refers to the actual quality of the electricity flowing through the unit. Clean power is free from "noise." It is free from surges, sags, distortions in the sine wave (IE something other than 60hz). Most sensitive electronics need a constant stable current that is free from noise or else they will fail quickly.
I'm not electrician I'm sure someone could give you better info but that is the jist of it.
For computers you want the voltage to remain at 12 Volts even as current draw changes, and, you want the electricity to be free of too much ripple.
The Japanese capacitors are not so much making a difference in ripple or line level, but they may be more resistant to heat, and last longer. Of course, a better capacitor can be made anywhere, they just cost a little more. It could simply be a matter of choosing to install a capacitor rated at a higher voltage that would make it last longer. Not better just spec'd higher.
As for the overall electrical performance it's probably more important that the design is good, and that the components are chosen to meet the needs of the design. It's when manufacturers use under-rated components that they get into trouble.
You can get a rough idea of the quality of a PSU by checking a) the max wattage rated, b) its efficiency curve, and c) the temperature at which it was rated. The higher the temp the better. Beyond that you have to test it out (i.e. read jonnyguru) to see how good the regulation really is when the PSU gets warm.