I'm asking in pure curiosity.
According to the tour, you shouldn't ask a question if it has:
[...] too many possible answers or that would require an extremely long answer
However, by the (short) time I've been there, it seems to me that such long answers are usually of great quality, because of the amount of relevant information it gives.
To answer this I'd like to provide a bit more context to that part of the tour. The quote you refer to comes from the section "Get answers to practical, detailed questions" and is under the part with the heading:
Not all questions work well in our format. Avoid questions that are primarily opinion-based, or that are likely to generate discussion rather than answers.
Questions that need improvement may be closed until someone fixes them.
The section you quoted then comes down to:
Don't ask about...
- [...]
- or any other question with too many possible answers or that would require an extremely long answer
Emphasis in the above quotes is mine.
This part of the tour seems to be trying to direct people away from asking questions that will be closed and the part you quoted is particularly trying to avoid Too Broad closures. The message there reads as:
Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question.
Taking all this information together it would appear that the quote is trying to avoid questions that will generate essay/tutorial length answers as they are not really a good fit for the question and answer model we use here. To quote an example of this from the comments.
That's not a long answer. Extremely long answers mean things like essays, which can be several pages long. Or questions which as for infinite lists. These are too broad. For example, "How do the themes in the Lord of the Rings affect how each individual character in the fellowship progresses through the story?" One could probably write thousands of words on such a question, amongst others. These are not appropriate.
As you can see the above example would generate answers that could be extremely long, it probably has multiple correct answers and will most certainly generate discussion rather than answers.
On a side note I believe the tour is mostly a fixed template, where the mods and community managers can alter select bits of the text. So whilst it's likely been tailored for our community, some bits of it could still be the standard text.
It is possible for some questions to require answers that are longer than the allowed answer length (which I believe is around 30,000 characters on most sites; a few sites may have special needs and allow 60,000 character answers).
If the required answer is too long for the site, then it's not a good fit.
However, by the (short) time I've been there, it seems to me that such long answers are usually of great quality, because of the amount of relevant information it gives.
I think that this looks at it the wrong way around. What you are thinking about is thorough answers. Thorough answers are great, as you said. Too Broad questions though lead to answers that are not thorough or in-depth but which are still very long.
An obvious example:
Please list all the people in the United States.
This is a ridiculously Too Broad question. Even without saying anything about any person, it's still going to be millions of words, much less characters. But this exhibits the problem exactly. Questions that ask for many shallow responses are not generally good questions for Stack Exchange. However, a similar question might be just fine for a phone book company.
