I read somewhere about how Windows introduced in early versions the ability to make buttons look "3D" (update: here: http://blogs.msdn.com/b/oldnewthing/archive/2004/07/28/19958...), and for the next few years every program made its buttons look more 3D than the previous version, until someone decided they had gone too far and started going in the opposite direction. And now looking at Metro I'd say we are at the other extreme (completely flat and minimalist). I hope it stabilizes somewhere in the middle in 5 years. Just look how fast they ditched the "vista" look.

It's funny if you look at http://stocklogos.com/topic/past-and-future-famous-logos. That page is satire from 2011... well check out the last "Microsoft" logo ;-)