A new study by Valve Software shows fewer than 8% of people connecting to their servers have been lured to Vista by Direct X 10 and only 2% are actually using it. The company president also blames the artificial restriction for a paucity of input devices for PC games:
Gabe Newell, president of Valve Software, said that Microsoft made a terrible mistake releasing DirectX 10 for Vista only and excluding Windows XP. He said this decision affected the whole industry as so far only a very small percentage of players can use DirectX 10.
... He would like to see controllers like the Wiimote or the Guitar Hero guitar, but since DirectX support for devices like these had increasingly been reduced over the last few years, developers didn't dare implement these expensive innovations.