Off the top of my head: Windows Vista was slow and unstable on a lot of hardware of the time due to significantly higher system requirements than XP and a new display driver model that worked poorly at first, had a very polarizing look, and had quite overbearing UAC -- where XP would just let you do the thing, Vista would ask you three times if you're really really sure you wanted to authorize it.

It had decent bones though -- arguably a lot of its bad reputation was due to hardware/third party driver issues and people trying to run it on old hardware that just couldn't hack it. Windows 7 was well received and is basically the same thing with small improvements and some of the UX issues smoothed over (i.e. less annoying UAC)