This actually is the reason I keep away from Xamarin, and I guess I'm not alone...
I think that there is a room for having a cross-platform lowest-common-denominator, including UI, File system, Networking, etc... with ability to dive into platform specific details when needed.
But it also supports the full experience for each platform.
At least, that's my understanding and OP (Miguel) confirms it. And I guess your mileage won't vary in this aspect ;)
Although, Xamarin.Mobile is a step in the right direction, as commented here.
Yes, but they do offer a "base" abstraction layer besides the C# libraries, that includes stuff like GPS access, accelerometers, etc.
As for the UI part, one could use a Webkit View as the view, and link the various heavy actions to C# code. For something like a common denominator UI, for a simple calculator or some form based stuff, it would be perfectly fine.