A few times when I have thought about flagging something I kind of wished there was a text box available to explain the reasoning for the flag. I could be tempted to check "other" just to be able to explain what I felt might be wrong (or to explain that there was some uncertainty).
(Part of this may come from personal difficulty with classification--that pixel seems to have a non-zero red value, so can one really say that the image is not red [not quite that extreme]--, but it might be helpful to the reviewer to have a short text clarifying the reason for the flag even if most people left such blank.)
I am posting this as a discussion question because it seems inappropriate to post it as a feature request on meta.stackoverflow unless it is something that actually would be useful to others.
(I am being lazy in not search M.SO to see if this has been proposed already. I may take a look later.)