When Vista shipped, the problem with drivers was that many manufacturers chose not to update drivers for most legacy hardware to the new, more secure driver model. The other standard complaint was with its security popups. Part of that was rough edges on the user experience design, part was just that people weren't used to thinking about security on their machines...that's changed over the past few years. And of course a lot of the complaints were because Vista was a change.

Also it's not that system was throwing UAC prompts on users, applications did. Ever since XP there were rules around what files go where, how installers should behave, etc. but people chose to ignore them (because they could). When Vista started enforcing certain things, elevation dialogs happened. What made MS in the days past (app ecosystem) broke it when quality started to matter.