Cogman is correct. There was so much bad information about Vista it was deemed bad without actual proof.
A lot of FUD was spread about how DRM was going to kill windows performace. The news was all a buzz about a paper Peter Gutmann wrote about how it was going make Vista's performance miserable. The mainstream news ran with it. Of course the problem being that Gutmann had merely read white papers on Vista's designs and made assumptions for the rest. He wrote his entire paper without ever having run Vista and never backed up his "research" with any actual evidence. Today it's easy to see how wrong he was as this very same DRM, the whole ability to play Bluray and use HDMI, is in Windows 7 as well.
What didn't help was that most hardware makers were lazy and slow about Vista drivers. With Vista MS required hardware manufacturers to have both 32 and 64-bit drivers if they wanted to have the Vista compatible logo on it. The result were late, poorly written or non-existant drivers for many devices.
Two Examples:
Video Drivers early on were bad. They caused Vista to have noticably worse framerates than XP did with the same hardware and settings. 7 months later, however, the very same hardware the problem was gone. All due to poor drivers. Several enthusiast sites proved it.
http://www.firingsquad.com/hardware/amd_nvidia_windows_vista_driver_performance_update/default.asp (for example)
Another example. I had an HP 1012 laser printer. A very common consumer laser. No 64-bit Vista drivers for years. By contrast, Epson had drivers even for ancient scanners. Suddenly though, when Windows 7 came out, there were drivers for both Vista and 7.
Vista's issues were all due to outside issues and incorrect perceptions.
I have a sinking feeling that Windows 8 might face the same perception feelings though, as people hate change and you know that both Apple and Google will work overtime fostering negative perceptions about Windows 8 since it will be directly competing with them in the Tablet market in a much stronger position than earlier versions of Windows.