First and foremost, I learned that people in Japan dressed like people in the West.
I had seen anime before, but most of the anime I had seen up until Sailor Moon were either pure fantasy, sci-fi, or really old. Even in things like Tenchi Muyo, certain characters did dress in traditional garb regularly, so I assumed in Japan, everyone dressed like that all the time. One reason why I thought Sailor Moon was actually set in America was because these girls, aside from the uniforms (which weren't that much different than uniforms I had to wear in America), usually dressed like American teenagers. Even Raye only dressed in garb when performing temple duties, which was strange given that I was used to seeing religious figures wear elements of their garments at all times.
I also learned that in Japan, they make new students stand in front of the class and introduce themselves. While this happens in some Western cartoons, too, this was something I never experienced in any of my years of schooling. Generally, if there was a new student, they took a seat, the teacher might mention it before starting a lesson, but for the most part they would
never write their name on the board, formally introduce themselves, or be presented like a trophy.
A lot of what I learned about Sailor Moon came from sites which explained more about the Japanese version.
I learned about the red string of destiny. Believe it or not at one time this common-as-dirt trope was completely unknown a North American audience.
I learned that the planets Mercury and Venus did not have moons. Before then, I legitimately thought Venus had a moon, since it was described as Earth's twin even back then.
I learned that the days of the week in Japanese not only correspond fairly well to the respective Sailors and their elements, but they also correspond very similarly to the days of the week in Spanish (
lunes,
martes,
mirecoles,
jueves, viernes - even if you don't know Spanish you can guess who corresponds with what).
While there is a similar connection in English...it's a lot less direct due to English's...ahem...tangled linguisitc roots.
I learned that "Chibi" means small. Up until then, the style known as "chibi" was known as "super-deformed" to me.
Thanks to the dub, I learned the words "
coup de grace" (specifically how it was pronounced) and "maudlin."