Feedback is an interesting beast. Because in theory I really like constructive criticism, and fannish circles do tend to disapprove through silence (which is both good and bad). But insulting the writer or making sweeping accusations about their personal character is not really constructive, and has little to do with the writing. Plus, there is a reason PMs were invented, so that there is a way to discuss critiques in a way that is less likely to publicly humiliate people or crush their spirits.

Really, in the end, like Zip said, it all comes down to having betas you can trust to keep you honest and help you improve. Great betas are rare and hard to find, but worth all the squee in the fannish yard.