The plan is actually remarkably straightforward. People who've installed Windows 8 preview releases will recall the installation spends a little while setting stuff up in the background after selecting a few user settings. During this part of the setup, Microsoft will play a short animation, explaining to both touch and mouse users how to activate the Charms bar - and that's it.
It's all quite elegant and simple, but it only explains this one tiny little thing of Metro - and yet, there's so much more stuff that's just hidden and hard to discover. I think Microsoft's hope is that by pointing users to one single gesture/location, users will be enticed to explore and discover the rest.
Other than discoverability, I think Metro's major issue is just how volatile it all is. Bringing up the Charms bar or task switcher is one thing, but getting users accustomed to just how easily they are dismissed is going to be a tough task. Current interfaces are relatively persistent, and things don't easily disappear without user interaction. The volatility of Metro's core user interface elements is something that still bothers me - and I've been using Metro every day for months.
Back in '95, this was enough to get users to use the Start menu. I'm very curious to see if this is going to be enough to get users to use Metro.