I think the hardest thing ever ever is the actual user interface.
It needs to be really well thought out and see how it caters to the intended devices. And then see how it could be useful on another device.

For instance, most bloggers say how much better Android is over iphones user interface (navigation, widgets, wallpaper, notifications etc) but when that is amplified onto a 8" tablet it just looks bloated and ugly.

If Nokia could get A+ with the entire user interface, it wont matter if they get C- in everything else, and thats the ugly truth. Because all the rest could be improved but when you try to patch the user interface, you notice that it starts looking very ugly (Frankenstein) and not a fluid experience. Because when you have a universal experience with the interface (ie built from scratch) it makes THAT MUCH of an experience, which is why the iPhone was so appealing in a world of HTC windows mobile ugliness.