The most helpful concrit I've gotten came when I was writing By My Side, and it was about characterizati on. BMS is my one big epic Nick romance, and I knew that in order to make it epic, I needed to break up Nick and Claire so I could bring them back together by the end of the story. There were some logical reasons for them to break up, but the way I went about it was all wrong. Never Gone had just come out, and I was in love with the song Siberia, and I was in my "song lyrics in stories" phase, so I decided to use the Siberia scenario for their break-up in the story... even though it went against everything Claire stood for as a character. She was set up as someone who said what she meant and would stand by Nick through anything, so there is no way she would have just up and left him with "just note left on the stairs." Except she did, because I made her. At least a couple of my more vocal readers were not very happy about that, and one of them told me quite bluntly what was wrong with that scenario. It was harsh, but it was honest, and it was true. I was probably kind of hurt by it at the time, but looking back, I know she was right. I learned a valuable lesson about being true to your characters and letting them take the lead instead of forcing them to do what you want them to do. I may be goddess of my little fanfic world, but I still want my characters to have free will.
Like Mare, I really haven't gotten a ton of concrit, and I can't think of anything that has just devastated me, but probably the most "hurtful" comment was someone comparing 00Carter to Agent Cody Banks LOL. Now, I haven't actually seen Agent Cody Banks, so maybe it was intended to be a compliment, but I don't think it was. I was a little bummed out by that because I love 00Carter. But I think it's easier to accept constructive criticism after the fact, for stories that you're farther removed from. Even though this was years ago, we're still writing 00Carter. BMS is old news by now, so even though I still love it, I can look back in hindsight and go, "Yeah, I shouldn't have done that. I would do it differently now."