Top 10 Windows and Unix vulnerabilities
SANS and the FBI have released a list of their top 20 computer security vulnerabilities, 10 for Windows, and 10 for Unix-based systems. Though the massive installation base for Windows PCs means that each vulnerability has a much larger potential impact, *nix isn't completely secure, either.
Inept or irresponsible admins who fail to keep their systems up to date with the latest patches aren't mentioned as specific vulnerabilities on either list, but perhaps the lists themselves can help get admins up to speed. Also missing from the lists are careless, negligent users, but I'm not sure how much can really be done about that particular vulnerability.