I don't think there is really a good way to approach "This is just like your phone, but better." WP7 is very different and there isn't much that can be shown as similar but more efficient that is hardware specific. Maybe focusing on the dedicated camera button, or the usage of the three capacitive buttons at the bottom would be a good idea? I really think Nokia should focus on the 900 and not specifically on the software for their commercials.
To show off the OS as a whole, I think they should not focus on hardware and should approach advertising the OS in a manner similar to Apple has done.
The Lumia 800 site does a good job of showing off the OS.