One of the biggest complaints Windows Vista receives is focused around the UAC (User Account Control). While touted as a security measure that would prevent a system from becoming compromised, most users simply see it as an annoyance, having to confirm even seemingly simple changes made to the system.
Microsoft has received a ton of feedback in this regard, and they seem to be preparing a simplified version of UAC for Windows 7 that will try to prompt the user less often.
Although early on Vista's life cycle many of the UAC triggers were prompted by software not yet written with the OS security features in mind, Microsoft admits they could have done better. According to some stats gathered by the software giant, one out of three Vista "sessions" create at least one UAC prompt. Not a huge deal when the machine is first being setup, but after it has been in use it tends to get in the way. Microsoft talks further details on its internal affairs with the upcoming OS on its Engineering Windows 7 blog.