A qualm-having FBvian points out a drawback of the Cash for Clunkers program (official name: Cash Allowance Rebate System), which is only open to vehicles manufactured in 1984 or later that get 18 mpg or less:
I have a problem with the CARS program – not all cars older than 25 years are “classic” cars. Folks with these “clunkers” are out of luck because of this restriction.
Near as I can tell, the ‘84 cutoff was put in partly to appease classic-car owners, who feared the feds might start confiscating or otherwise messing with their older rides.