Gimmick is just a feature that makes your product stand out. One that increases its appeal.
In gaming, when used negatively, it means something that does not contribute to how games are played.
While I would agree that HD, motion controls and 3D are gimmicks (in the good sense), I think they all add something to games.
Lots of gamers nowadays won't play new games on SD TV if they have an HD TV. The picture quality helps to immerse you a bit more.
Motion controls are not gimmicks in the negative sense because they can change how games are played and make them more fun and immersive. It all depends on whether the dev tack it on or take advantage of it.
As for 3D, there are already people on gaming sites (NeoGAF and such) that say after they've played certain 3DS games with 3D on, they can't play the game without it because it makes it feel as if something is missing in the game.
We're going to see some nice things done with 3D on 3DS, I'm betting you. I already think it can add immersion to games, and that's somethign important.
