Explanation of Content Guidelines
Answer the question actually asked.
This can include addressing a more general problem—if all or at least most of the answer also applies to the more specialized problem. In some cases, answering the question can include answering a question of which the original poster was not aware.
Do not simply post your opinion; show your reasoning.
Not only does this allow the community to evaluate the validity of the answer (which incidentally will encourage readers to upvote the answer), but it can also help the original asker recognize how appropriate the answer is to the particular problem (which may not have been precisely communicated) and can help readers answer other similar questions.
Unlike many sites, Stack Exchange sites are not looking for opinions and discussion. Answers should be focused and open to independent evaluation of the evidence provided.
Provide a sufficient but not necessarily exhaustive answer.
For some questions, even a book might not be long enough to provide a complete answer. Providing links or references to additional resources can be helpful for addressing such complex questions.
For questions with less subjective answers (concerning, e.g., citation standards or copyright law), references should be provided.
For questions with more subjective answers, it is sometimes possible to reference the opinions of recognized experts. However, be careful to choose appropriate experts (e.g., a writer of action-adventure scripts might not be a good expert on clarity in technical documentation, despite being an excellent writer) and not to take such expert opinions out of context.
Include relevant quotes from references rather than just a link.
Aside from issues of link rot, forcing the reader to follow links to reach the answer is less helpful. An answer should be complete on its own.
Provide links rather than just citation information when possible.
Making the reader work harder than necessary to access additional information or confirm the accuracy of a reference is not helpful. When recommending a book, linking to a book seller is acceptable and can be helpful. Links to amazon.com will be converted to use Stack Exchange's affiliate program (as suggested here), so providing amazon.com links may help fund Stack Exchange.
Do not be afraid to edit your answer.
Stack Exchange sites are dynamic and collaborative. An answer might go through several revisions. This is normal. Do not be afraid to edit an answer to increase clarity, correct mistakes, and add new information.
Since edits to an answer bump the question to the top of the recently active questions list, it is generally preferable to make fewer substantial edits. However, it is generally better to correct mistakes and clarify substantially confusing writing sooner rather than later.
Embrace the collaborative ethic.
Others may edit your answer to clarify what you have written, correct simple errors (grammar, spelling, word choice, etc.), provide links, or otherwise enhance your answer without changing its essential content. Understand that they are trying to help.
Similarly, comments may be posted to your answer seeking clarification, suggesting additional content, or pointing out problems with your answer. In general, such should be taken as encouragement to improve your answer. Clarifications and other enhancements should be place in the answer, not in a responding comment.
Even when a comment expresses disagreement, it may be appropriate to clarify what is being stated or to strengthen the supporting arguments. Sometimes disagreement is unavoidable, but often it indicates a misunderstanding or a perception of an unsubstantiated belief.
Do not be afraid to incorporate content from comments (or even—to a some extent—other answers) into the answer. When such content is included, provide attribution to the source. Including a link is generally encouraged for references to other answers. Comments (being more ephemeral) are not as conveniently linked, so not providing a link is not problematic.