I recently spent a goodly amount of time composing a PM and forgot to fill out the subject line before clicking on submit. Because I forgot the subject in the PM, it made me use my back button and everything I had written was, of course, lost.

Why not a reminder that a subject line is missing rather than a "one of the required fields is missing" statement that loses the content of your PM? The reminder, like email hosters ask, could say "Are you sure you wish to proceed without a subject?"

I was REALLY disappointed to have lose the content of my PM because I forgot a subject.