One thing I'm not exactly thrilled about with Windows 8 is the two-program-max multitasking. I know you can have a ton running in the background, but I use between 4-5 while I'm working on a website, all of which I like to have open at once.
Also, I wish that you could choose to have the start screen extend over dual screens, instead of one side being desktop, the other being start. The way it works in the developer preview is weird, hoping it's changed in the consumer preview.
Obviously, since it's only a little menu. I just think it's weird that they're saying "the desktop is an app, not the main interface," but then forcing dual monitor users to use the desktop as if it's the main interface.
I'm really not a fan of this new UI, they're considering tablets over first and foremost Desktop PCs. Priority should really work like this: Desktop -> Laptop -> (Phones ->) Tablets (although the final two are interchangeable).