Could we have a method of explaining why we think a question or answer should be deleted?
It could work identically to closing a question, maybe with an additional text-input box (like the "Requires Moderator attention" flag option) to provide additional info because the reasons for deletion should really be more specific..
For example, "exact duplicate" doesn't explain to me the user has re-posted the same question, thus, it should definitely be deleted (not just left closed). Like with close-messages, these reasons would be listed under the deleted item (or answer).
I don't think comments are suitable, as they can be a pain to quickly.. "extract" deletion reasons from them - I have to scan through the comments, and they may all be irrelevant.
Equally, for un-deletion, it'd be useful to know why another user thinks a question should be restored. For example "new user deleted the question after they accepted an answer", or "Jeff went insane and started deleting any questions with the letter O in it".
I think requiring un-deletion reasons is possibly more important, since commenting is disabled on deleted questions - we have no way of explaining why we think it should be restored