The "big new thing" is the new Metro user interface which sits on top of the classic Windows 7 desktop:
|The Windows 8 Metro UI|
It makes the PC look very like an Android tablet or iPad - very modern and very slick, but pretty hopeless if you want to do some "real work" with the usual Office products such as Excel, Word, Outlook etc.
You can drop into the classic desktop view to run these programs as usual but there is something obvious missing... the Start button! This means that there is no easy way to access your program list, the Run dialog box or - most importantly for me - the instant Search box. I use the Search box to find and run programs, find emails, find documents... and there simply isn't an equivalent in Windows 8.
It looks like I'm not the only one to find the lack of a Search box a pain - http://www.extremetech.com/computing/96249-5-deal-breaking-flaws-in-windows-8/2
It also means that it isn't obvious how to switch the computer off either - in fact, it is so obscure, I gave that as a test to Dan and Kim and they failed just like I did...