When reviewing low quality posts, the Recommend Deletion option has bunch of optional comments, like "Link-only answer" or so.. But sometimes none of the provided options reflect the real case.
It may be good to provide a way to leave a custom message, like we have in flags.
For example, I saw couple of answers which were just one or two lines of code, like this answer.
When flagging, we have the other option which allows to flag post for reason not covered by common options. I just thought here it may be useful too, for the rare cases which happen from time to time.