I come from a iPhone 3G, the first wow was the screen quality of the N900.

Its just so much more crisp and clear than the iPhone display.

The second wow, the battery. In comparison to the iPhone.

The third and for me; most important wow (can you even say that :P ) - the Maemo system. This is just fantastic, compared to the os 3.x of the iPhone.
And of course the possibilities of multitasking on the phone, the display, the browser - there are just so many nice things about this device.