[Note: the following is a rambling of notes I jotted down quickly after viewing the new Windows 8 release.]
The one feature that I truly dislike is the use of panels. I originally thought they'd be an interesting and welcomed change for the desktop. I was wrong. They do nothing but hinder easy switching between apps and viewing of the start menu. They are big, clunky, and not suitable for a desktop at all.
The second thing that causes a major headache is the lack of familiar options. For example, to shutdown, the user has to click Start, then the "Settings" panel, the "Shut Down." This is not intuitive at all.
Finally, the color scheme is horrible. I know it can be changed, but the default teal / green is a shocking step backwards in Windows' appearance.
Check out the quick screencast: