By far the most remarkable thing is how much of these changes stop existing when Explorer isn't running. Technically I like the ribbon in Explorer because I'm a fan of the ribbon UI for cheesy HCI activism reasons, but I don't think I'd ever actually resort to it since right-clicking is more immediate.

The technical differences are as follows.

A new framework for building Metro-style applications, and a collection of applications that run on them.

Removed applications like Windows Mail that were redundant with Metro applications—except IE, which exists in both forms.

Explorer: removed start menu, added multi-desktop taskbar, added start screen, added win-tab toggling, added hover overlays, added controls for managing docked Metro-style applications, added new ribbon interface.

Removed some icons from network control panel. No more park benches!

Added font size controls to Display for weird task-oriented reasons.

Split language and region control panels.

Replaced Windows Defender with Security Essentials.

New task manager. (Voodoo digital signature crap prevents it from running on Windows 7.)

Renamed "Parental Controls" control panel to the far more patronizing "Family Safety".

New, fancier session manager (lock screen, ctrl+alt+del, etc) that looks like Metro but is completely independent from the explorer and application components.

New visual style and sound schemes.

Broke support for Aero glass. Not compatible (?) with .msstyles from earlier versions.

Forgot to update UI of Windows Media Player. (What the hell, guys?)

So as far as I can tell, if you're not using Metro, the list of reasons to switch to Windows 8 is unusually slim. I guess theme designers will begin targeting it exclusively, so I could see modders getting possessive, but it really, really, really feels like an internal build or a service pack in terms of how little has changed. It's like the difference between Windows 95 and Windows 95 OSR2. Maybe they think they're approaching perfection?