I propose that a good question meets the following criteria:
- The answer to the question is not easily found in the first results page of a Google search.
- The question is focused on Drupal
- Not critical of Drupal
- Answerable outside the context of the specific case
To support the first bullet point, you can Google any of the suggested questions that Mark through out as easy examples and see the first few Google results cover them easily.
To support the second point, I think it's obvious, but you will inevitably get questions such as, "How do I set up a LAMP stack on Ubuntu so I can run Drupal?", and this should not be discussed here but rather over at ServerFault (however, a question such as, what steps can I take to make Drupal 6.x run on Ubuntu 10.x. would be welcome).
The third point is subtle, but questions such as "Why is Drupal slow?" are just detracting. Questions should be more focused and worded so as to promote Drupal usage, so a better question would be "How can I improve the performance of Views in Drupal 7?"
Finally, for the fourth point, there are sometimes questions that come up in #drupal IRC where the person will basically say, I'm doing something really crazy, they cannot share the code, and they just want a quick answer to move on. I think the value of Drupal Answers on Stack Exchange will be to discourage hit and run Q&A, and rather focus on substantive answers that will be beneficial both to the original questioner and answerer, as well as to many future searchers down the road.