In terms of big selling points, you can hardly get better than the DS' touchscreen. At first, no-one really 'got' it, and it drew a lot of criticism and mocking, but it didn't take long after release for people to realise that the power of touch was a pretty interesting hardware addition.
A little while on, and we've got even more innovative hardware with the Nintendo Wii, but neither the touchscreen nor the Wiimote seem to have been used to their fullest. We've had handwriting recognition (Dr. Kawashima's Brain Training), platform drawing (Kirby Power Paintbrush), and soon we'll have world-making (Drawn to Life).
What's missing from your touchscreen wishlist? And, uh, keep it clean.