Only ask what you care about
When stated explicitly, this may sound obvious. Do not ask what someone cares about, but ask what you care about. If you do not care about a question, either no one really cares or those who care can ask a better question. Posting a question which you do not care about is a waste of time for everyone who reads the question (and possibly tries to answer it). Put differently: Do not ask a question for the sake of asking a question.
A corollary of this is: Try to answer by yourself before posting a question. If you really care about a question, the first thing to do should not be posting the question on a website, but trying to solve it by yourself. This includes looking up literature, formulating it from different angles, considering small cases, special cases and/or variations, and so on; in short, what you normally do to solve a problem. If you do not want to think about the question by yourself, ask yourself whether you really care about the question.
This does not mean that you must not ask a question unless you have proved that you cannot answer it by yourself (which is obviously impossible). However, make some reasonable effort to try to answer your own question before posting it.
Curiosity is a great motivator for science, but you should still make an effort to solve a question by yourself before posting it.
- Even if you care about the question, the reader may not know it. Therefore, it is a good idea to include some evidences that you care about it in the question. Doing so also helps the reader to understand the question better and hopefully results in better answers. For more on this point, see this guideline and this guideline.