At this moment in time, windows 8 almost feels like you're dual booting two different operating systems at once.
When you click through to the desktop, it's everything you're used to from windows 7. There are some tweaks, such as the removal of aero glass, the new office-like ribbon menus in windows explorer, and the way you access the power options by mousing over to the top right of the screen, but nothing that will really phase you. The only difference is that the start menu is gone, to be replaced by:
Metro UI. This covers all the same functionality your start menu used to; you can pin shortcuts, or start typing to search for files, settings and programs. However, the big difference are the 'apps', for which microsoft have launched their own walled garden app store. When you launch an app, it fills the entire screen - your taskbar isn't visible. Metro apps are clearly designed for tablets and phones, with touchscreen control in mind. You can only switch between apps using an awkward mouse gesture at the top left corner of the screen (something that is actually really tough when using dual monitors). At the moment, they are all very basic in terms of functionality and customisation. I'm interested to see what they make, but currently, they are useless for users on a PC.
So, because I have yet to find any use for Metro or the Metro Apps, windows 8 is really just windows 7 with a fancy, full screen start menu.
EDIT: I just realised that Alt + Tabbing shows a combined list of metro apps and windows, so that's easier than messing around with that gesture based system of switching.