For example, while watching a movie (using MS' own player, even...though their player doesn't support Blue Ray, you have to have a separate piece of software for that) any notifications in the bar will popup and interrupt the movie. Given that notifications have become a constant feature of the OS in the past few years (really, does a mouse driver need to have a popup to notify you of updates?!?!? apparently so...likewise Java, likewise Windows updates, even Steam, which I want to like, is a nuisance). Notifications are happening all the time, and they'll interrupt anything else you're doing. I've never liked the notification bar in Windows, and I've always hated how many pieces of software think they're important enough to be allowed to nag me constantly while using the machine (it's one of the reasons I would never used Windows on my desktop), but Vista seems to take it to a new and astonishingly horrible level.
In the same vein, screensaver and power saving do not know when you're watching a movie...so, you pretty much have to turn them off, if the machine is for watching movies.
In a different direction, the audio, network, and video drivers keep breaking. Not sure why or how, but every few boots I have to reinstall one or more of the drivers from the original disks (installing via Windows automatic detection and installation installs drivers that don't work), or from the manufacturers website.
Also on that subject, audio will not work after changing the input on the TV--it uses HDMI for the audio connection, and some types of "off" events can never be recovered from (while others can...actually turning off the TV and turning it back on works fine...but switching inputs and then switching back breaks it).
Shutting down is broken. How can that be? I have no clue. But, sometimes when the box is shut down, it reboots immediately after. Not always...but about 40% of the time. Sometimes it goes into a state where it can't be shutdown properly, and a hard power off is the only solution.
While I have lots of other complaints about Windows, even when it's working exactly as intended, these things are just bone-headed and irritating, and can't possibly be considered right by anyone sane.
I keep thinking the next Windows Update will solve one or more of these problems...but, so far, they're all still present. The audio thing was fixed for a while, but then it broken again. I hate mysterious behavior in computers perhaps even more than wrong behavior, so this just served to piss me off more.