That you have to use okra to call it gumbo. That thread originated on the OT so not sure it should count.
You could list any number of the gumbo "truisms" on here.
Gumbo has to have okra
Gumbo can't have okra
Gumbo can't have celery
Gumbo should only have onions, not celery or bell peppers
All "New Orleans" gumbo has tomatoes, and gumbo shouldn't have tomatoes