Here's one of the more intriguing tidbits in the hundreds of pages of emails released as part of the Vista "junk PC" lawsuit: A Microsoft exec freely admits that users are so annoyed with User Account Control (UAC), that they're turning it off en masse.
Biggest thing I'm worried about [concerning Vista uptake] is UAC (user access control). It looks like more and more people are turning off (based on advice from websites) for easier friction free use.
By the way, notice that even Microsoft execs can't remember the proper name of this feature -- it's user account control, not user access control.