I've been part of the Windows Vista testing program since the days it was still called Longhorn. I have to say, they've come a long way in that time (a what a long time it was!). All in all, Windows Vista seems to be a good step forward from Windows XP - although there are going to be a number of things that annoy the seasoned Windows user.
1) Wireless Networks - At the moment, there is no way to tell an adhoc network from an access point. This means it's potentially easy to trick people into trying to associate with an adhoc network and not what they are actually wanting to connect to. This becomes more interesting if the same SSID is used between the AP and the attacker - as Vista will prefer the stronger signal - which may be either of the two networks.
2) Windows Sidebar - I am yet to find a good reason to use this. It's slow, limited and steals screen real-estate without adding much usefulness. Indeed this would have been better implemented with something more like OSX's Dashboard - however even an option just to hide the sidebar off the screen when the mouse is not near it would be a great improvement. Thankfully, it can be disabled easily.
3) The Start Menu search - This is one of my pet hates. After years of being used to pressing Win -> U -> U to shutdown the computer, this no longer works. If you press the Windows key then type 'u', you now end up searching the start menu, favourites (bookmarks) and web history. While this may not be a bad feature, it's annoying when you've been using that key sequence for years!
4) User Account Control - Although I understand the reasons for Microsoft to make this move - making a more secure system by trying to separate system permissions from user permissions, the UAC pops up all over the place - from simple things like defragmenting your hard drive to changing font DPI. The worst part of this is that the UAC prompt is modal - meaning that you MUST pay attention to it when it comes up - you can't just do something else and ignore the prompt.
There are a few more slightly annoying things, however in the whole, things seem to work ok. My test machine for Windows Vista is a 2 year old Dell Inspiron 8600 laptop with a 1.7Ghz Pentium M CPU, 1Gb of RAM and a Radeon 9600 video card. The performance rating is 3.5, however it's still easily quick enough for everyday use - even when using the Aero theme.