Here's my first attempt at writing the FAQ. Please note that I've marked this answer as "Community Wiki", so feel free to edit it to add, remove, and improve and answer.
What kind of question can I ask here?
Salesforce StackExchange is for Salesforce.com and Force.com users, admins, and developers. It is run by the Salesforce community, for the Salesforce community and, while Salesforce.com employees participate, it is not governed by Salesforce.com in any way. If you have a question about...
- Salesforce.com CRM application
- Force.com platform
- AppExchange app development
- Mobile app development using Force.com SDK
- Integration with any of the above using one of the Integration options
and it is not about...
- Mobile App Development in general
- Cloud Computing (Saas, PaaS) in general
- Soliciting help to write test suites for your code
- Soliciting help to write actual code for your given requirement
...then you've found the right place to ask your question.
What kind of question should I not ask here?
You should only ask practical, answerable questions based on actual problems that you face. Chatty, open-ended questions diminish the usefulness of our site and push other questions off the front page.
Your questions should be reasonably scoped. If you can imagine an entire book that answers your question, you’re asking too much.
If your motivation for asking the question is “I would like to participate in a discussion about ______”, then you should not be asking here. However, if your motivation is “I would like others to explain ______ to me”, then you are probably OK. (Discussions are of course welcome in our real time web chat.)
To prevent your question from being flagged and possibly removed, avoid asking subjective questions where …
every answer is equally valid: “What’s your favorite ______?”
your answer is provided along with the question, and you expect more answers: “I use ______ for ______, what do you use?”
there is no actual problem to be solved: “I’m curious if other people feel like I do.”
we are being asked an open-ended, hypothetical question: “What if ______ happened?”
it is a rant disguised as a question: “______ sucks, am I right?”
(The above section was adapted from MetaFilter’s FAQ. For more detail, see six guidelines for great subjective questions.)
If your question is about the site itself, please don't ask it here. Visit our meta-discussion site where you can talk about things like what questions are appropriate, what tags should be used, suggest a feature, point out a bug, or generally discuss how Salesforce SE works.
If your question would be of interest to …
Professional and enthusiast programmers, ask on Stack Overflow.
System administrators and desktop support professionals, ask on Server Fault.
Power users of web applications, ask on Web Applications.
Enthusiasts and power users of the Android operating system, ask on Android Enthusiasts.
Perhaps your question would be a better fit on another Stack Exchange site; see the site directory for details.
What is SFSE?
I think everything else can be copied verbatim from the Super User FAQ, since it is applicable to this site as well. The important bit is the first part - what kind of question can I ask here that needs more attention and thought?