Re: They're sort of there...
This is for WinRT apps only, so the common API is present. You can already produce universal apps for tablet and phone; the core logic stays the same, but you usually have to tweak some of the UI views.
There's no need for spaghetti code: in the (rare) cases where logic differs from one platform to another, you create a (small) interface that's implemented by different concrete classes, one per platform - hardly meddling in the Dark Arts.
Users don't care what version of Windows they're running. They care whether the software they want can run on their device. The big change here is nothing to do with the APIs - it's that you can now run those tablet- and phone-friendly Metro things in a desktop window -- something which should have been possible from day one of Windows 8.
Now, I wonder if Apple will allow iOS apps to be run on MacOS X? (no, I'm not being serious - there's a huge security problem if they did)