I think the public forum is the place for the things you liked about a story. The things you didn't like should be addressed in private, unless they are very minor and easily corrected.
Things like pointing out plot holes, mistakes in facts and things like that can be very helpful, if embarrassing. These are questions of method...and method is what learning to be a better writer is all about.
Things like objecting to someone's politics, though, is less effective. It might be ok to ask if they were intending to convey a certain message with their story. Maybe they didn't. Things like theme and subtext can be unconscious in the writing process.
But if you don't like the message they are sending, then criticizing them isn't going to be very helpful. It's like criticizing someone's politics or religeon...they'll get defensive, and you probably won't agree.
Of course, positive expressions of what you'd like to see in the future parts of the story are more helpful. Sometimes they can spark ideas in the author that he hadn't thought of before.
Sometimes the author was already planning on what you are asking for, so then he or she can be inordinately pleased with themselves. <g>
But overall, major criticisms of a story are a little like having a boss criticize your job performance.
You might do better in the future, but you'd much better hear it in private.
That's my take anyway.
