Japanese language is very tightly integrated with Japanese society, to the extent that I would cheekily argue it doesn't even count as speaking Japanese outside of the context of a Japanese social setting or structure.
To elaborate on your point regarding the proliferation of vocabulary from honorific synonyms, consider how much of the grammatical gamut is wrapped up in identifying the speaker's relationship with the listener. Who is the in-group? Who is the out-group? Is the speaker a man or woman? What is the relative social position of the speaker and listener? Is the utterance written or spoken?
Japanese is a language that identifies the answers to all of these questions with explicit grammar markers, while omitting number, case, conjugation, and employs only two tenses (realis and irrealis).
Learning Japanese in a classroom outside of Japan, or heaven forfend, through individual study alone, is an awkward and disconnected experience, since there is scarce opportunity to practice the major components of Japanese grammar in a real way.
Also kanji.