1. An 80s high school is a very dangerous place. There are big gangs of karate-fighting bullies all over the place, and the school officials will just watch and not stop them from beating you up.
2. 80s high schools regularly employ paramilitary groups just to hunt down students and get them in trouble. Because, you know, a regular hall monitor is just a sissy.
3. There are no police in the 80s. Everything is street justice. Somebody tries to kill you? No cops, have to learn karate to stop them.
4. All elderly Japanese men sound like they have been chain smoking for like 90 years.
5. In Japan, people speak broken English to each other instead of Japanese, even when the flighty American kid isn't around to need to understand them.
6. If you choke some kid in public, everyone will just stare and not do anything about it. Because, you know, choking kids is perfectly legal in the 80s.
7. Underage drinking is okay as long as it's sake, not beer.
8. 80s high schools conveniently have bird cages on the roof in case you find some injured hawk you want to hide or something.
9. Mr Miyagi is the only karate teacher in the known universe who isn't an arrogant jerk.
10. Maintenance guys at seedy apartment buildings can afford really awesome housing. Also, apparently once they start teaching Karate for free they don't have to show up at work anymore.
11. Being a karate instructor is exactly like being a mob boss. You get your own posse who pushes everybody around so you can pretty much own whole towns.
12. Japanese thugs kick harder than American ones.
13. Rich guys have nothing better to do than get revenge on complete strangers.
14. Shop doors in California obviously do not lock. Otherwise how would Daniel be dumb enough to leave it wide open for the bad guys to get in multiple times?
15. No seriously, there were no cops at all in the 80s. Nor were there insurance companies, so if your stuff gets stolen, you're screwed.
16. I stand corrected. There are cops, but they laugh at you when a crime is committed instead of doing their jobs.
17. Nobody can hear your evil laughter if you do it behind a wall.
And of course, the most important of all:
Don't mess with the old guy. He can kick your butt.