I want to ask a question on Japanese Language and Usage about X. Is it on topic?
Disclaimer: opinions differ somewhat about what questions should or shouldn't be allowed. Also, this answer is a work in progress - I may have missed a few meta posts or examples of closed questions.
Some questions are regarded as off-topic for Japanese Language and Usage, but are permitted on meta: Allow limited resource questions in meta? . This happens especially with questions on input-methods how to type Japanese. An example of such a question is How can I learn to type on a JIS keyboard?
Questions that are purely about Japanese culture are off-topic: https://japanese.stackexchange.com/questions/2511/is-it-a-rumor-that-eating-really-loudly-is-considered-polite
Questions that might summon Cthulhu are off-topic: https://japanese.stackexchange.com/questions/374/regex-match-sentences-except-sentences-contained-in-「」
This question has been superseded by What questions are not allowed on Japanese Language SE? and How should I format my questions on Japanese Language SE? , which are both tagged as faq.