There was a Facebook app calling itself Honesty Box that attempted to fill this niche a while back, although it had its limitations (an interface discouraging long replies; only allowing one question at a time). So it's been tried. No one on my friends list has used it for a couple of years, though, and nothing equivalent seems to have arisen.
I suspect social networking's about the best place for such a thing: asynchronous, readily anonymized, sufficiently impersonal but not too much so, and structured such that people with access usually won't be tempted to give malicious feedback. On the other hand, limiting the answer pool to acquaintances introduces some bias -- but that's a difficult bias to eliminate short of establishing strong reasons for giving feedback to strangers, a solution unlikely to arise in any casual setting.
LW probably wouldn't be much worse, with weaker access controls but stronger etiquette norms and a tradition of accepting short-term discomfort in service to long-term goals. The lack of face-to-face contact outside of meetup groups is a weakness, though; you can glean quite a bit from someone's behavior over a text channel, but as a group I think we're already pretty well practiced in communicating that way and would benefit more from feedback regarding other channels.
Most people need feedback in many areas. Most people can give feedback in many areas. But for some reason I don't see a lot of actual honest feedback happening, neither in my personal life, nor at work, nor here on LW. This looks like some sort of market failure, or perhaps a bug in society.
Would we benefit from a norm that encouraged asking for feedback or critique in any area, perhaps using open threads set up specially for that? I think we would. What do you think?