Blame Intel and
collusion.
More sealed documents were released Nov. 14, and they further suggest that monopolies Intel and Microsoft colluded to reduce the graphics standard to accommodate Intel's 915 chip set.
[...]
Throughout much of 2005, Microsoft worked with OEM partners to establish clear graphics guidelines for Vista. OEMs knew about the WDDM requirement, which the 915 chip set didn't meet. If Intel continued selling these chip sets, without Vista support, some OEMs would push back (and not take orders). Public acknowledgment that Intel's main graphics chip set wouldn't support Vista posed potential risks for the company on Wall Street. At the time, everyone expected Vista would be ready for holiday PC sales. So, it would be crucial for OEMs to ship Vista-ready PCs throughout as much of 2006 as possible. If 915 wasn't ready, OEMs would have to use different graphics chip sets.
[...]
According to a Feb. 27, 2007, Microsoft e-mail: Only 60 percent of graphics accelerators shipping on desktops or notebooks supported WDDM during fourth-quarter 2006; 30 percent were Intel 915 chip sets. Seventy-four percent of notebooks and 58 percent of desktops shipped with integrated graphics. Eighty-six percent of notebooks were eligible for a Vista logo, but only 46 percent could run the Aero user interface. Consumers got the logo but not necessarily the fully implied promise.
Moneyshot!
This explains the "varied" performance experienced with Vista by some MS employees and other MS customers in a certain community I know back when Vista was released.