It would be good if people could leave you feedback on your CV. This way someone who came across your CV could say if something struck them as particularly off putting, or if it had a typo, etc. I think this would be a good selling point to job seekers.
It might be good from the job seeker's point of view, but employers have to be very, very careful on giving feedback of any kind to potential employees both for legal reasons and to avoid prolonged discussions with people you frankly will never hire.
It's one of the reasons you'll never hear, "Your portfolio showed lack of ability in area X, Y, and Z" - would be awesome, you could deal with those things and come back two days later and try again.
But from the perspective of the employer it's unnecessary work, and that's assuming they haven't exposed themselves to a lawsuit depending on how they said, "Not at this time, thanks."
It's also the reason they rarely tell you at the interview that it's a no-go.
Companies like Fog Creek where you can get that kind of feedback are rare.
Still, this is a different career website - I would like to see the feature implemented, and see what comes of it.
It would probably have to be anonymous, and employers wouldn't be able to see comments other employers made on your CV (ie, they'd be for your eyes only).
I just suspect few will actually use it, especially since a resume says so very little about an applicant - at best it would be mechanical (spelling, grammar, did/didn't meet requirements, was a worse match than the other ten people that showed up on the search, etc.)