Due to several discussions on the main site, I am asking this question concerning the best practices for tagging questions on MathOverflow. Unfortunately, the relevant documentation page https://mathoverflow.net/help/tagging does not answer the following questions (except partially questions number 2 and 4 below). Moreover, that help page is not specific to MathOverflow.
Ideally, the result of this discussion would either be added later to the documentation, or it would be turned into a FAQ thread. I hope I am not overstepping my bounds here.
How should I tag a question on MathOverflow? Which tags are most helpful? Which tags should I not use?
What makes a good tag?
What are top level tags, and why should I try to use them? [It would be a good idea to list all the top level tags here, and perhaps their meanings as well.]
What is a meta tag, and why should I not use them?
Why are some common tags deprecated?
Please feel free to add new questions, or to modify the ones above.
I hope this thread eventually leads to a sufficient consensus, and that its content can be a group effort. If the above are too many questions, perhaps we can separate them into several meta threads.