American full contact vs low-kick rules
One rule separates them, and 1988 showed exactly what it is worth. Everything above the waist is identical.
American full-contact kickboxing permits strikes above the waist only. Low-kick rules are the same code with leg kicks added. That is the entire difference on paper, and it changes the sport more than any other single-rule difference on this site.
Under full-contact rules a fighter has no reason to defend the lead leg, no reason to keep weight distributed for checking, and no cost to standing tall and mobile with the lead leg forward. High, frequent kicking is rewarded — American rules even mandated a minimum number of kicks per round, an obligation unique in combat sports and imposed because bouts kept collapsing into boxing.
Bill Wallace's career illustrates the point. He retired undefeated as a champion while throwing almost exclusively left-leg kicks, having no functional right kicking leg. Under a ruleset where anyone could attack his lead leg, that career is not available.
Rick Roufus against Changpuek Kiatsongrit in 1988 is the demonstration. Roufus was an outstanding American full-contact fighter, was ahead on the hands, and lost because his lead leg stopped working — against a technique he had never in his career been required to defend.
The honest reading is not that American rules were inferior. It is that a ruleset determines which skills a fighter ever builds. Roufus was not outclassed. He had a blind spot that was invisible from inside his own code, and the only way to see it was to fight under a different one.