1. Navigation blocked because an issue with the security certificate or other stupid warnings.
   Here we can see that I even need a couple of additional clicks even to check my Adsense earnings:
   
2. You don’t really need an XP upgrade. What does Vista does that XP does not?
   There is really no need to waste money on a Vista upgrade. There are good linux distributions like Ubuntu that will do much more than Vista and will make you save money.
3. Vista automatic upgrades : As default when you switch off the PC it makes you wait until installing all the updates. You don’t really want this.
4. Driver support. It just _sucks_ and there is still 1 month I got this Dell with Vista and I still couldn’t make bluetooth work. I’ve read that Nvidia is working furiously to get a stable driver for the 8800 out by the 30th and also Creative’s popular DirectSound based EAX no longer works at all. That among some popular things that don’t work. With the same laptop I booted the latest Ubuntuin another partition and everything was working out of the box (even bluetooth).
5. Very bad usability. Alerts eveywhere each time you want to do anything. It really makes you feel a stupid user with warnings like : Do you really want to […] The software XXXXX is trying to […] So you are constantly making clicks to confirm all this alerts making the user experience quite disgusting. This is the famous Vista stupid allow popup that asks you every time you want to do anything!

Apart of all this and more you can read here and here this Windows version is definitively the worst I ever tried.