The core operating system swap. Taking a desktop operating system and shrinking it down to run on a phone is no easy task. In fact, it's a gargantuan undertaking. Add a bunch of new end user features and you get that the amount of technical work that is going into WP8 is phenomenal. However, the windows phone engineering team is superb. I have no doubt's that both consumers and developers will be thoroughly impressed come holiday season.
Taking a desktop operating system and shrinking it down to run on a phone is no easy task. In fact, it's a gargantuan undertaking.
iOS is a shrunken version of OSX, while Android is a barely shrunken version of Linux. Aren't all modern mobile platforms shrunken versions of desktop operating systems?
Who is to say those weren't gargantuan undertakings as well?
The relationship between Windows Phone 8 and Windows 8 is closer than that, though. My understanding is that both will make calls to the same WinRT APIs, meaning a significant amount of code (and the XAML defining the UI) can be shared between both platforms. That's a very different relationship than Mac OS has with iOS (where the UI differences are significant) or Android has with Linux (where there is almost nothing in common between X and Android's UI).
