Let's use this meta question to collect the already-vetted explanations of common reasons for questions to be put on hold. The intention is that you'll use a comment (suggested here) to link to a specific answer here when voting to close in an applicable situation, if nobody's already done so.
Because we're going to be linking here and many readers won't already be meta regulars, please don't add lots of comments to the individual answers here. And please don't answer here to propose something; there are separate meta discussions of several close reasons or risk factors, and this post is intended to collect the final results of those.
Here are some comments you can use, with links to these answers:
Too broad -- too many questions
Asking multiple questions in one is not recommended as it makes it hard to answer with a concise and focused reply. Additionally it becomes much harder to rate answers as to whether one is better than another as the "best" answer to each part of your question may be held in different answers. See [tips on how to fix the problem](http://meta.worldbuilding.stackexchange.com/a/3364).