“WHAT could be so damning on a birth certificate?”
The information that reveals he is not a natural born citizen.
IMO IF the date stamp we can see that bleeds through from the back of the 'Certification of Live Birth' presented on the fightthesmears website - IIRC it was June 6, 2007 - is the date the document was 'produced' THEN ANYTHING THAT MIGHT HAVE HURT HIS CAMPAIGN WOULD BE DAMNING!
Anything at all that negates the carefully constructed myth of 'Dreams From My Father'.