I participate in a site where people are very sensitive about... well, everything. I've had comments of mine being deleted ever since I can remember, and I used to be ok with that because a) we need to keep things civilized and b) comments are ephemerous at best anyway.
Today however I noticed a case where a potentially constructive comment was removed as rude. Some context: OP was asking about things that could cause political instability in a fictional world. Another user commented something along the lines of:
You could have a look at the general history of pre-WW2 Europe for a real world example of such a scenario.
That is not the comment verbatim. I don't have access to it because I am not a moderator, and my memory is not perfect. Maybe it was really rude, but let us suppose that it was written exactly as above.
Also, I see how some people can view this kind of commenting as an error. It does not ask for clarifications and it does not suggest improvements to the question; rather, it is an answer in the form of a one-liner. The user who made the comment could have posted a proper answer. But that is not what I'd like to talk about.
In a meta discussion in the site, a moderator said that the comment was deleted for being rude, and not following the "be nice" policy. Fair enough. I am okay with deleting comments if they are seen as rude. But what is rude to a group of people may be customary to another group. Having your comments deleted without warning or notification does not help bridge that gap.
So what I propose is a multipart feature:
When you have a comment deleted for being rude, you would receive a notification like:
One of your comments was deleted for being rude:
[Original comment here]
Please remember that in order to participate in Stack Exchange, you need to observe our policies about being nice to other people. Be welcoming and patient, especially with those who may not know everything you do.
Or some similar text, and the link would point to the same help section but for the relevant site (specially for localization in the case of non-English stacks).
As an extra, moderators could have the option to add some text explaining what in the message was rude. The user would receive some notification as above, but the moderator in charge of the deletion could optionally also type something like:
Hello John Doe. I understand what you meant here but please notice that [some expression here] is considered condescending in many places. Please try to communicate in a way that is more friendly.
Please notice that I have never been a moderator; I don't know if moderators can already do this when contacting a user, so if that is the case, please ignore this part.
Comment Review Queue
Last but not least, it would be nice if regular, non-mod users could handle flags on comments. This could take some load off moderators, and call the community's attention when someone is actually being rude. This could be a queue for users with 10,000+ reputation, which is generally when you are trusted enough to have access to moderator tools.
I have seen a similar question: Could we provide some automatic feedback on (enough) unfriendly/rude comments? But that one is about warning when a user has had too many comments marked as rude so that they think before commenting. This one here is about letting people know exactly what comments of theirs are being considered rude so that they can learn about improving their comments.