There have been a couple of questions of of the form "tell me everything about topic X". See for example:
The first question got a number of great answers. Still I have a feeling that questions like this are too non-specific to be answered in a QA site and their answers are likely to be elaborate enough and require large enough effort that they would do better as a page on Wikipedia, if such a page doesn't exist already.
Also I feel that the askers of such questions are being lazy by not attempting to read Wikipedia/books and trying to learn things on their own before making demands on others' time. If they don't know where to start then their question should ask for references.
Mathoverflow FAQ says "Mathoverflow is not an encyclopedia". Should we adopt the same principle here?