Most people just seem to "know" that Vista is a terrible OS. The read some forum post that heard that it was terrible, or they saw some random news story a while back saying people weren't too happy with Vista. I've even heard some people claim it was Windows Me all over again.
Then you have some of the "advice" people give out for Vista users. My favorite, of course, is that the first thing a Vista user should do is completely disable UAC. Now, sure, those pop-ups can be annoying, but I'd prefer a quick second of annoyance when loading up a program over executables starting off silently with admin privileges.
In any case, I've used the Windows 7 beta as my main OS for a couple months now and haven't had any issues so far.