He allowed IE6 to languish while the competition was growing.
To me, Ballmer is symbolic of Microsoft-with-ADD. They ignore things until they become urgent, and then shovel money into the furnace until victory comes. Look at Windows Phone - microsoft had a mobile offering when iOS came on the scene, but took years to move on with WP7. Desktops languished on IE6 for far, far too long.
And Vista's long death-march process and lackluster result is another point.