I can't speak for others, but though every kind word on a story of mine is
cherished and kept for posterity, it is to those who DON'T like my writing
to whom I owe a debt of gratitude.  A very fine example is one well-known
figure of internet anime (who shall remain nameless, as he has no e-mail
over the summer, and cannot defend himself) who called my characters
'cardboard' and complained about their 'absolute lack of characterisation',
the artificiality of my dialogue, the lack of background for my stories, lack
of humour in my jokes and the predictability of my plots.  His very thorough,
very specific letter inspired me to do better, and I hope you'll agree, if
you'll go through my work, that it has improved in those areas.

Did he post this message to the mailing list so he could have something to do? 
Telling an author something is wrong is one thing, doing it in public is 
another. Reminds me of a review Kergma did for a fic a while ago. He said that 
it 'bordered being a parody' that all the characters acted wrong, and that the 
add in character's ways of joining the Knight Sabers were...pathetic. This was 
all useful information, but had he posted it on the ML I would have, um... 
disliked him.