I don't know if there's any final definition, but to me, the difference between a genuine innovation and a gimmick is staying power.
For example; let's compare the Wii's motion controls and the Nintendo DS's touch-screen.
While it's by no means some kind of universal edict, most gamers get tired of waving their arms around like a fool inside of the first thirty minutes of gameplay. Remember when Wii Sports was new? Everyone was really into it, using their entire bodies to play the game. Now, it's just considered annoying.
The touch-screen, on the other hand, has been accepted and even praised. Games like Kirby: Canvas Curse and The Legend of Zelda: Phantom Hourglass have shown us that the touch-screen and stylus combo can, indeed, have a place in games that isn't annoying or intrusive, but a natural extension of gameplay itself.
Of course, what it really comes down to is how the element in question is implemented. I'm right there with other gamers who claim the Wii's motion control is mostly a gimmick, but I have to admit, when it's used in games that try to work with motion controls instead of shoe-horning them in, they work well, and can actually be kind of fun.