Unsolicitated feedback: Between people seeking revenge for real or perceived slights, people who don't believe the feedback is fair, lawsuits over same, the site would drown.

Plus who would sign up to enable others to have anonymous feedback about them? If you are really good you would get hired without it, so it would be a sign that, at most, the person is a solid B-player and likely a C player. Would you hire such a person?