Actually finding more uses with the touch lately than I did when I first had the DS and DS Lite. The drawing programs on DSiWare and 3DSWare make good use of the stylus considering you can draw with a stylus. The most common use of the dual screens is basically just having it so you don't have to switch to the inventory/map. Not the most dramatic thing but it helps keep the action going. I never mastered TWENY using both screens so I usually had the top screen on auto. Still need to finish that game...
I like the 3D a lot and I use it most of the time. Probably 25% of the time I play with it off to conserve battery life or I am laying down in position where I don't care to hold it for the sweet spot (or if I don't have my contact lenses or glasses on since I can't see 3D really that well without corrective lenses). 3D effect isn't a total game changer but there are already games out now and soon to be released that show that it can add some cool effects (Super Mario 3D Land and I believe Luigi's Mansion 2 will be very nice with the 3D on).
To answer the main question of the thread... DS - wouldn't have sold as much without touch, 3DS - the inclusion of 3D has only helped its sales slightly
