There are a lot of reasons for the failure of Windows Vista, but in retrospect the biggest reason was that the OS simply didn't matter that much anymore. Most of the consumers who ended up with Vista simply got it because it came installed when they bought a new computer. The vast majority of them never chose Vista.
The group that did have a choice with Vista was businesses and they chose to avoid it, although not because of any inherent inferiority of Vista. Vista has been very usable since Service Pack 1 and since vendors finally updated their software and drivers to work with it by early-2008. The problem was that there was never a compelling reason to upgrade to Vista. It was the software equivalent of repainting a room and rearranging the furniture.
Now we have lots techies singing the praises of Vista's successor, Windows 7, which will be released later this year. I just got finished testing Windows 7 for two months. I used it as my primary production machine at the office every day. I installed it on a high-powered 64-bit Hewlett-Packard desktop machine. I loaded all my apps on it. It worked fine. However, my conclusion on Windows 7 was, "So what?" There's nothing in Windows 7 that matters. In fact, the computer operating system has never mattered less than it does today.