Flagging comments, I ran into this:
I'd pasted-in a reasonable reason, but the Flag Comment button was disabled! And, the dialog/UI didn't provide a clue as to why!
Eventually, I found this brilliant request for a reasonable number of characters for the field. (2047 characters would be a fine choice. (^_^) )
Meanwhile, there is an easy, immediate, UI fix... Merely add a single bit of static text to the dialog, like so:
IMPORTANT!
The comments shown are not the final comments I flagged with (obviously, since they were being rejected). People commenting that those comments could have been condensed (after minutes of hindsight) are missing the bulk of the point of this question.
That is: that the UI should indicate why an input is being rejected.
I won't rehash the length argument anymore, since most people have already resoundingly supported that notion here.