Although if you are in a PAL country and your player isn't setup right US NTSC discs can look really really bad as setup/pedastal whatever you want to call it can end up being added twice, resulting in a really washed out picture.
Basically the Japanese discs have an extra 7% or so of range to play with, so more detail in the shadows etc. Also, the Japanese laserdisc owner was considered a more discerning viewer, so extra effort was usually put into Japanese releases to squeeze the best quality out of a transfer.