Downvotes and close votes should always be accompanied by a comment. While not strictly required as per SE policy this is especially important for new users. We should direct our criticism at bad or unwanted questions, not at unwanted users. Especially beginners – no matter how otherwise knowledgeable or qualified – make mistakes, that is to be expected.
Because there are a lot of similar situations, below is a list of comments that can be copy-and-pasted to provide new users or repeat offenders more insight into how this site works and give reviewers a little help to save on keystrokes.
We adopted the stance that comments are our barn cats. Very useful for improving a post, but also easily gone. Re-use of comments is difficult if they get deleted.
Obviously, self-written comments are always favourable, but because we are often lacking comments before closevotes and downvotes come, and really canned close-reasons are limited, here are a few suggestions that should provide a backbone for standard situations. These should not be copied blindly but preferably adapted to the individual case, if possible.
Please edit the existing answers and add more examples as answers and as needed.
A list of "magic-links" to save on typing and character limits is on meta.