What does a good JL&U question look like? What are the qualities that a perfect JL&U question would have?
Guidelines for answering this meta-question:
State assumptions that you may have made. This helps to either confirm your suspicions or for the community to correct your assumption.
In order to get higher quality, more focused answers, it's often better to ask unrelated questions separately.
For example, asking "What's the difference between 愛{あい} and 恋{こい}? Also, what's the difference between ごめんなさい and すみません?" in a single question is less likely to get targeted, high-quality answers than if it was split into two separate questions.
It's also sometimes a good idea to split up a question which is very broad in scope into multiple, more specific questions.
Knowing what you want to find out. Do not do a generic "I don't understand everything in this sentence" question. Dissect the situation and isolate the problematic portion.
Can be useful to people besides the asker
Questions that are general and could help many people are better than too specific ones that only help the asker. For example, if part of a song line confuses you, it's better to ask about that part than to ask what the whole song line means. Maybe that confusing phrase is used elsewhere in Japanese too, but I doubt that the whole song line is.
I think this question was deleted because I can't find it now, but within the past 6 months, there was a question asking how to translate a list of very specific phrases for a flyer. I didn't even know what some of the things meant in English. This is not a good question because nobody can use it besides the asker.
Here's a similar example (although not as bad as the above question): What does 女子高生に唾液かけた疑い 逮捕の男「困る姿楽しい」 mean? This question will help very few other people besides the asker, if it even manages to help others. A better way to create this question would be something like "How is the Japanese in newspapers different from textbook/conversational/normal Japanese?" This could help a lot more people than asking what the quote from the article means.
Evidence of some form of independent research prior to asking
There's nothing wrong with asking beginner questions that seem "easy to answer" to many here. Everyone has to start learning somewhere. But, at the very least one should consult Wikipedia and/or a dictionary and/or a search engine as appropriate before asking. These are important research skills anyway, for anyone who is serious about learning a language. If you don't find an answer (in a language you understand), then think about asking here. If any research you have done seems relevant, it should be included in the question.
Has enough context.
A sentence or phrase etc can have many meanings depending on the context, so if relevant, please outline where you saw the text and provide any relevant contextual information.
In the case of phrase requests, stating how you would like to use the phrase as well as the phrase itself can often be a good idea as there may be multiple ways of translating depending on the context.
Providing information that could possibly help you find what you want to find out. This means that after highlighting the problematic portion, show what the surrounding context may be. This includes your "working steps" - like for example if there are issues that you suspect is specific to one instance, list other similar instances that you think are fine and show the contrast. Conversely if there are many instances where a concept appears incorrect but there is a exception, list the instances where they appear incorrect and show the contrast.
Have a logical framework. Build your question properly; have a clear direction as you write/type. This means arranging your ideas (isolation of the problem, providing surrounding context and stating assumptions) where it would be most sensible to put them - either group them together or have a consistent logical flow. (A,A,A>B,B,B>C,C,C or A>B>C>A>B>C>A>B>C)
Format the text to improve readability. This means using line-breaks, horizontal rules, lists, blockquotes to section off parts of the question. A question typically has these elements:
Preamble
The problem
Hypothesis
Evidence and (counter)examples (Either textual, graphical or may be in a list)
Do make the question such that it is easy for to determine what each section is at a glance. If the question is rather complex, do not keep writing in a continuous paragraph without any form of delineation between your thoughts, your examples, your opinions, facts et cetera.