Drivers (either bad or non-existent), a long wait from the previous release and an over-generous dose of hype....
If I remember rightly there was an MS board member that wrote a steaming letter to Steve Ballmer after he put the freshly minted Vista disk into his home PC and turned the hub of his family's 'digital lifestyle' into a 'f---ing email machine'.
HP readily expected my parents to go out and replace their 1 year old laser printer with a new one as they were not going to provide Vista drivers. Luckily HP's XP drivers came with an install routine that was so badly written that the necessary dlls would get installed correctly before crashing.
Edit: hang on a minute... In Vista RTM, network file copy was totally crippled. I remember being shocked when I had to copy my music around for the first time. Also, there was a screaming bug with explorer file views. Some folders would get a special view setting (Music management, picture management) tattooed to the folder. Impossible to change. These were shocking bugs to go to production and force on millions of users. It was enough to convince me not to bring Vista into my work environment.
I think we're in danger of looking back with the rose-tinted spectacles and blaming the industry press for negative reviews. Vista RTM had shockingly bad quality issues, high real-world hardware requirements and took five-ish years to deliver but only represented two years of development work.