Wii mote added a lot to gameplay, especially in Skyward Sword and FPS, and it actually changed gaming.

Gamepad is still to early to tell, but Pikmin 3 makes really good use of it, making it easier to control different captains. Zombi U too, thanks to the scanner (but this is possible without it), and looting/using your backpack without pausing or having a little pop-up in the middle of the screen. Also, off-tv play, nuff' said.

3D... it just makes games look better. So IMO, 3D is the most useless.

The Wiimote makes certain games possible. The GamePad frequently adds to games. 3D, on the other hand, barely is ever very useful and drains battery life to boot. So 3D loses, even though I like it and the 3DS.

The analog stick.

Tie between the GP and 3D output. Both nothing more than developer vanity.

The Wii Remote made it impossible for me to go back to DA for tps/fps. Hence the codename; Revolution

3D in the 3DS. There's a reason there's a 2DS :)

FPS on the wiimote was soo fun. And in my opinion its the Gamepad, still not sold on it, mainly because I can't mulitask whenever I try it I forget about the gamepad screen