There is an issue with Metro that has presented since the first leaked release candidates with multiple paths to failure; most typically if you have created a local account (not used Microsoft credentials to create an account), and then used a Microsoft Metro application that requires Microsoft account credentials you're account store becomes corrupted... I've just confirmed (first hand) this issue remains in Windows 8 RTM and Server 2012 RTM...

In the best case and most typical scenario this presents as the inability to launch Metro applications, but will also always break your access to the advanced Metro 'Change PC Settings', and most importantly your ability to use the system restore features including access to 'F8' boot option which now boots to a Metro interface rather then the text mode recovery options of Server 2008 R2, Windows 7, and earlier Microsoft Operating Systems...

The only documented 'fix' Microsoft has on offer short of reinstalling the OS, has not worked for everyone, and is almost as onerous as you have to create a new account and delete the broken one... Just what this procedure leaves dangling in the wild in it's aftermath remains to be seen but there are anecdotal reports of unremoved shares, and obviously you may loose account specific data...

Searches relating: 'unable to launch Metro applications', 'Metro apps won't start', and lost access to the Metro 'Change PC Settings' interface will turn up a lot of anecdotal reports; the only place Microsoft appears to be documenting this at this time is here on the TechNet Forums and here on the Microsoft Answers Forum... This seems like a rather egregious over sight and to corroborations of McAfee's claims Windows 8 has larger attack surface may lead to even greater vulnerability...