For a product to be successful, it's necessary (though not sufficient) to be able to define it in one sentence.

The iPhone is: "A touch-screen phone that's also an iPod".

The BlackBerry was "Push email".

The Wii is "Game console with a motion sensor".

The Motorola RAZR was "Insanely thin clamshell".

The Titanic was "Unsinkable luxury ocean liner".

The Walkman was "Music on-the-go".

The N800/N810 are "Internet on-the-go".

Nokia N95 is "Maximal convergence device".

Nokia 5800 is "Comes with music".

Unix is "Everything is a file".

The idea needs to be in place before the product is designed. Once you have the "single big idea", the rest of the product design more-or-less falls into place around it.

If you can't convey the idea of a product concisely, not only won't it be possible to market it successfully, but the product itself will probably bomb because it lacks a sense of purpose and will end up being designed by committee.

For the N900, "Full internet in your pocket, always connected" would do it for me.

Regards,
Roger