NOTE: This idea may be a big flop or maybe I didn't organize this very well, but...
A few years (decades?) ago there was a book published titled ‘Cultural Literacy.’ I may be misinterpreting the book’s goals, but I believe it basically tried to catalog important bits of cultural knowledge that every American *should* know. In every culture there are historical facts, cultural nuances, idioms, etc that are common knowledge to those within that culture but may be little known to people from other cultures. Even with cultures with the same base language (for example the US and England) there are major differences in cultural knowledge.
What I would like to see here is a collection of words, phrases and commentary on topics most adult Japanese should know, but wouldn't be common (textbook) knowledge among non native speakers. Most entries wouldn’t be found in standard dictionaries, but may be often referred to in Japanese novels, TV or in conversation.
I am sure this will help learners understand hidden nuances easily missed by the otherwise diligent student of Japanese.
This may include bits of knowledge from any of these categories:
• Japanese History (not obscure history, but common knowledge) • Japanese Mythology (which may include myths from other cultures taught in school) • Idioms • Pop culture icons (Doraemon to SMAP) • Technology • Household & Family • Fine Arts • Religious references (words that have entered the common language or references to ceremonies)