It's just PR - a new interface is a new OS, when underneath it's Windows CE - a new version maybe, but Windows CE ultimately.
Until apps are recoded to use the new "Windows Phone 7" interface APIs (and there's no SDK that includes them yet), third party applications will be using the ancient Windows CE user interface controls still. Or they won't be compatible. And thus the device will have few third party applications available at launch. And given that developers will be looking at the sliding Windows Mobile marketshare today, they won't rush to get their apps running on Microsoft's idea of a touch interface.
It's based on WinCE but with the UI thrown out, so existing CE apps aren't compatible. Though I'd be amazed if they changed much of the core CE APIs.
New UIs will need to be WPF or (again I'm presuming here - based on XNA compatibility) a DirectX subset.
So although it's new it will still be accessible to developers who have had exposure to Windows development which, to me at least, seems to be just about everyone.
You're not going to see a thousand fart apps on day one, but they will come with time.
That said, based on the Win Phones familiarity with DirectX and Microsoft's relationships with the game development studios I wouldn't be surprised to see some big name games pop up early in its life cycle along with a lot of ports from the independents on Xbox Live.
In any case we're in for an interesting year!