Vista is EEEEEEEVVVVVVIIIILLLLLLL!!!!!!
First you have 10, yes 10 different versions of Vista:
Windows Vista Starter Edition
Windows Vista Home Basic
Windows Vista Home Basic Upgrade
Windows Vista Home Premium
Windows Vista Home Premium Upgrade
Windows Vista Business
Windows Vista Business Upgrade
Windows Vista Ultimate
Windows Vista Ultimate Upgrade
Windows Vista Enterprise Edition
For the regular consumer, they confronted with the list and are left scratching their heads trying to figure out which one to buy.
If you start digging into Vista itself you run into a bunch of stuff that is just plain annoying such as User Account Control. It asks "Did you initiate the process that's attempting to run?" Unfortunately it's dumb. It doesn't try and figure out if what you are doing is really a threat just that something might be exploited. Somethings that can cause this to require you to authenticate: Opening Disk Defragmenter, and Adjusting font sizes. It is trying to protect you from yourself.
The upgrade versions of Vista are a poisen pill. You used be beable to wipe your hard disk and insert your previous Windows CD into the drive for validation of ownership when the installation process prompts you to do so to reinstall the system.
Under Vista, the ability to insert a previous copy of Windows in the CD drive for validation has been removed. Microsoft has provided an alternative route to achieve the same goal, but more advanced users are apt to consider it to be less convenient. You have to have the previous version of Windows already installed on your PC. Then, when you run Vista's setup, it will give you a clean-install option.
What if your hard drive failed or you got the urge to install Linux on it? Well you now need to reinstall XP first before you reinstall Vista. If you move your upgrade installation to a new system, there probably will come a time when vendors stop making drivers for XP, then you're stuck buying a full copy of Vista later.
I could go on with this, but all in all Vista is evil.
At work, I use Windows, Linux, & Mac OS X. For the first time ever, someone came in and said I want to get a new computer what should I get? I actually told them, if you can get WinXP a Dell should be fine for their needs. If they can only get Vista, buy a Mac.