You won't find a Homebutton on it anymore and will probably be running Meego (Linux).
It's made out of one solid polycarbonate body, precision manufactured, as they say. The screen is slightly curved and seems to be around 3,8" or 4". Here's a video of the UI in action:
As you can see, it performs rather well overall, but the maps in 3D seem to lag a little.
The absence of a main homescreen is something I personally would have to get used to first, but I do like the feature of just swiping an application away.
With the N9 running Linux, there's a chance that it will be hacked to run Android too!
There's no infop on the hardware specs yet, but I'm hoping it's not 680Mhz ARM... again...
A dualcore would be great for better multitasking, gaming and general performance.
Here's a video with Marko Ahtisaari on the N9:
Here's a video with Marko Ahtisaari on the N9: