A semitone flat on the high notes.
A square with only three sides.
A steering wheel / few bolts short of a Yugo.
A teapot with a cracked lid.
A titanic intellect... In a world full of icebergs.
A vacuum-tube brain in a microchip world.
A VGA card and a Herc monitor.
A victim of retroactive birth control.
A violin minus the bow.
A walking argument for birth control.
A wind-up clock without a key.
About half smart.
Afraid she'll void her warranty if she thinks too much.
Airhead / bubble-brain.
Aliens zapped him with stupidity ray -- twice.
All booster, no payload.
All crown, no filling.
All foam, no beer.
All hammer, no nail.
All hat and no cattle.
All he remembers about his middle name is the first letter.
All his eggs in the same basket.
All his learning curves look like Mount Everest.
All icing, no cake.
All lime and salt, no tequila.
All missile, no warhead.
All of his bytes are odd.
All shot, no powder.
All the lights don't shine in her marquee.
All the personality of linoleum flooring / plasticene / putty / caulking / saran wrap / a bowl of oatmeal / a plastic spoon.