It feels like the Harmattan team at Nokia really understood good UI and good design, and wanted to build a product around those ideals.
The N9 is also a really slick piece of hardware. Someone designed and loved this thing, and wanted it to be amazing. It doesn't have any weird bulges or outcroppings like all the Android handsets; the appearance of it is minimalist; even the camera module on the back looks like it fits the style. I always found the Android cameras to be just another annoying outcrop, never fitting with a cohesive design.
The N9, and Harmattan, have that cohesive design. It's a royal shame that it's the best phone no one will ever use...