I tried to flag a comment on SO as "unconstructive" since that's a valid reason now according to the tooltip. Of the options available, "too chatty" seemed like it might be a good fit, but I wanted to explain my reasoning a little more, so I selected "other..." and typed in my explanation. The "Flat Comment" button is disabled and clicking it does nothing. I have 28 flags remaining. If I selected one of the other radio buttons, the button is enabled again. Seems like "other" just doesn't work.