The trains running in the subways are much newer, cleaner, and wider than the ones in NYC or Tokyo or Paris or London (based on my own observation).
They also continually build out entirely new rapid lines like the Bundang express line which takes you from a suburb of Seoul to the center of Gangnam (a major district) in 15 minutes.
Their buildings are also mostly new. It seems like every decade or two Koreans demolish older buildings and replace them with new ones. Whereas the US and Japan seem more often to keep old buildings around for much longer (and don't clean them).
The cars on the street in Seoul were also newer than the ones I saw in Japan (although similar in age to NYC). For example the taxis in Japan look like 80s BMWs. They look very cool for that reason but also seem pretty old.
Finally in Japan there's a lot still done by cash and weird single-purpose ticket machines. It would be a hardware hackers dream because of the variety and number of ticketing machines everywhere.
But in contrast Korea has gone almost totally cashless everywhere or so it seems. You just "tap" with your phone. The US is getting there, and maybe Japan is now too since it's been a few years since I was last there.
I wrote a bit more about this a while ago (and included pics) here: https://notes.eatonphil.com/on-nyc-tokyo-and-seoul.html.