A gimmick is something that has little to no real-world usefulness. A gimmick isn't defined by the object itself, but rather, what uses it has. Anything labeled as a "gimmick" at one point in time, could become a fundamentally important feature later on, depending on how it's used.
Let's take Sixaxis as an example. On the face of it, it seems pretty useless: You can tilt the controller to do things. How is this useful? What's wrong with analog sticks or buttons? However, there is one very useful thing that I have used for Sixaxis for in games: Aiming.
Gravity Rush, Uncharted GA, and Killzone Mercenary (along with some others) on the Vita give you the option to use Sixaxis for aiming. I've actually found myself using the motion sensor almost exclusively for aiming in these games, as it's far faster and more accurate than using the analog stick.
I don't think gimmicks are bad, as long as they don't add a significant additional cost to the item (Kinect says Hi). People will usually find something useful to do with it.
"Never argue with stupid people. They will drag you down to their level and then beat you with experience."
-Samuel Clemens
