"As for the 90% thing, that's total hyperbole. The WinRT libraries and Win8 UI will have a learning curve, but you seriously underestimate how much of software development is knowledge and experience with logic, design patterns, OOP concepts, mathematics, data store manipulation, IDE/Tools, unit testing, debugging, best practices, and years of other basic computer science stuff."
I'm talking about the curve needed as someone tries doing things that they've been doing for 10 years, but can't in WinRT (at least not without re-learning)... like reading a file from disk, or storing application settings. There are entire swaths of functionality that are either missing by design, missing due to it just not being a shipping product yet (that category is, thankfully, small), or significantly changed (usually to work with the new paradigms like async operation). Anything involving UI is "throw it out and start over".
For someone who's been in the WinForms world, other than foundational knowledge (best practices, the language syntax itself, any libraries which happen to still work without modification), nearly everything they know at a "tactical" level is invalidated by a move to Metro/WinRT.
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