Here are some examples of good feedback:
I felt a bit lost in the first four paragraphs because I can’t tell the killer is in a car, can’t figure out if he actually just killed someone, and don’t get much visual on Lorna. Is she his ‘kindred spirit’? I have absolutely no idea, and while the vagueness might stir me to read on later in the story, opening so ambiguously, without grounding me in a real scene, leaves me restless.Why is it good feedback? It's because as a reader, they explained what they didn't understand. What didn't make sense to them. 'Vagueness' and 'ambiguous' are things you want to change in your novel.
‘A friend recommended this book to me,’ - I thought it odd she explained it this way, given the text in the books would be the same, wouldn't they? Maybe if she came up with something on the fly that distinguished that particular copy from the others, it would be less apt to garner attention from the library worker. She is going through such a series of events to get to the appropriate copy, I'd expect her to cover better.Why is this helpful feedback? If a character does something out of character and your readers pick up on it, that's wonderful. You don't want the people who buy your book to make the same complaint because it will cost you money and readers when it counts most.
How can we give good feedback?
1.) Explain what you liked about the piece. Be specific. ("I liked the monologue on page three," "I liked the way the main character handled the situation," etc.)
2.) What elements of the story did you like and would like to see more of? In other words, what parts can be expanded for your enjoyment and/or understanding? ("I think the mother's dark sense of humor is really intriguing, and I would like to see more of that," "The relationship between the father and son has some good conflict, and I would like to see more of that," etc.)
3.) What confused you or what didn't you understand about the piece? ("I'm confused about when this story is supposed to take place," "I don’t understand why he walked back in the room when he said that he wasn't," etc.) This includes perceived technical/logistical problems. ("The war that your main character is referring to was in 1812, not 1813," "One of your characters looks out at the sun and comments on it, but earlier in the piece someone says it's midnight," etc.)
These are some of the things important for the writer to hear so that they can improve.
Source: How to Give Feedback to Writers
Sidenote: Right now Harley D. Palmer from Labotomy of a Writer has been doing a series on Characters. She has had some really interesting posts.
1) Epic Character Questionnaire - A 253-question interview you can ask your character. It's extremely in depth.
2) Character Development Before and After - How your character has changed over the course of the novel.
3) Character Details - showing little things about your character.