My all-time-fave is a comment I got on a story that no one ever reads.
OH MY GOODNESS YOU'RE AN AMAZING AUTHOR!!!! It made me so sad to realize I finished all that had been published, I really really really hope you continue this! It's so perfectly written and you capture the characters thoughts and feelings amazingly!! I'll be waiting right here for your return to this masterpiece!!! 
I did finish it and notified this reader, but they never replied back or left more comments. Maybe it was too late. Lost their attention. 
I wish readers would realize how happy these comments make authors, how motivating it is. Whenever I think that a story is a failure, I take comfort in those few nice comments. It's not a failure. The story is just right to a specific audience. That's all. Can't please everyone.