Should a user have to add a comment when they vote to close
I'm very surprised that this has not been asked in this manner before. (Did I miss it? Search didn't display what I was looking for.)
Incentive for this request is a question of mine on Programmers that was recently closed, but I distantly remember multiple similar cases in the past, too.
Questions currently get closed with a general tag defining the issue (like 'off topic'). Usually there is also a comment explaining this further.
Sometimes this comment is missing, and the user may well be left in confusion. That's why I would strongly encourage everyone and especially mods to actually leave a comment explaining the problem.
This avoids a fair part of the 'why was this closed' kind of questions on meta. It is also polite, helpful and constructively gives users a better understanding of what the site is about.
Referring to the FAQ is not at all sufficient. It is well known that users don't read messages, and while I do understand that everyone is required to get themselves together and try to improve the site with useful content that conform to the statements made in the FAQ, making users go all the effort to study the details closely enough to understand cases that are not blatantly obvious is certainly wrong.
It also displays an unfriendly and impolite attitude, and while the majority of users won't care or mind too much, some do feel a bit offended and repulsed. This is really easy to change and provides significantly better user experience, so why not?