I believe that it is very important to make the phrasing essentially positive, so as to encourage questions as much as possible from the main population of people that might make valuable contributions to MathOverflow. We need in particular to phrase things so as to encourage questions from graduate students who might otherwise feel inhibited to contribute.
In my comments on Will Sawin's answer, with which I am very largely in agreement, I had suggested something like the following:
In order to achieve a good reception for your question, perhaps the most important thing to do is to explain it well, with mathematical accuracy and precision. Provide explanation for or definitions of the mathematical terms or notation that you use, in light of the fact that people around the world do not always use the same terminology and notation that you may have learned. Describe the mathematical context in which your question naturally arises. Mention the mathematical techniques that you think might be relevant for a solution.
Perhaps one could hint at the idea that this is a site for sophisticated mathematical users by saying also something like:
This is a site for mathematically sophisticated users, mostly at the PhD-student level and above, and so do not be inhibited to explain your mathematical ideas with full technical detail.