Eh...I've seen some DS games that use the touchscreen well, but it's stuff like Etrian Odyssey where game mechanics (i.e. the map system) are designed specifically for the touchscreen - the majority of games use it as either a way to half-bakedly adapt a genre not suited for the existing controls (i.e. FPS ports) or an annoying "gimmick" mechanic dropped into the middle of a completely unrelated game.
Of course there will be bad examples, but on the whole I loved the touch screen on the DS.
I'm in agreement with Yahtzee: requiring larger motions for the same actions detracts from game immersion, it doesn't add to it.
I liked Resident Evil on the Wii. Using the WiiMote in the gun housing is really fun! Also, House Of The Dead is a great arcade game when you've got a few mates and gun housings for all your WiiMotes

And I'll put this one on clever marketing designed to accomplish exactly that goal (showing the general public games as a normal form of entertainment, not trying to offer condescending induction into the "gamer culture" cognoscenti.) And the price difference 
Yes, marketing is another factor. XBox and PS3 are marketed as hardcore, gamer-oriented devices, and the Wii is marketed as a more family-friendly, casual gaming device. Surely the fact that this marketing was effective says something for the effectiveness of the control mechanism, since this is central to the marketing campaign of the Wii.