I have found that posting a position on monster.com generates 100's of
applicants but it is hard to tell them apart.   Why don't applicants
have feedback ratings like ebay?  This would be really useful as I
have been burned by people with nice resumes and checking references
is such a hassle.

Of course, the rating should be reciprocal. 

So many employers are erratic, never follow through, change their
minds, cancel the job, don't pay well, etc. It's so hard to get real
information about companies, especially when they hide behind
fly-by-night recruiters. ;-)

I do think the ratings should be reciprocal.

Why don't any job boards do this?

Further to czh-ga comment , 97 to 98 per cent of IT jobs advertised on
Internet job search firms go through RECRUITMENT AGENCIES for Europe.
If you do not believe me check any of the following Search tools 
MONSTER, JOBSERVE or www.reed.co.UK. 
  
REED is itself a large recruitment company (possibly the largest
recruitment company in the UK)
Yet for the vast majority of IT jobs listed ON ITS OWN INTERNET JOB SEARCH
site go through OTHER recruitment agencies.(Again IF YOU DO NOT
BELIEVE ME CHECK FOR YOURSELF)
Furthermore often the SAME job is posted through multiple agencies (Thus 
internet search sites grossly overstate the number of actual position
which are really on the market)
Which bring me to my final point Why do employers wish to hide behind
recruitment agencies?
Mongolia