I think in gaming, the term "gimmick" is basically something that is heavily marketed, claimed to add so much value to a platform or revolutionize how games are played, but ultimately adds nothing to the overall gaming experience and ends up being nothing but a bullet point that no one actually ever uses. Using them could be generally more trouble than they are worth as there are better or easier ways to acomplish the same things.
Not Gimmicks.
- Analouge sticks. They are not the same as buttons, and certain geners today would simply be impossible without them.
- Vibrating controllers. But they add immersion to the gaming experience and a new layer of sensory feedback
- VR. Still in its infancy, but this has the potential to change a lot of things about gaming or entertainment in general. As long as you don't mind the added isolation. Which honestly most people won't mind anyways. Contrary to what many think, I personally believe that its movies that will be the VR killer app and not games. At least initially. (can elaborate on this if you are curious)
- Controllerless Motion gaming. Been around for a while (contrary to what MSmay want you to believe) and first became a mainstream ting on the PS2. It sounds cool on paper, but in practice results in all round unresponsive and very limited gaming. And when added to actual controler based games, is usually nothing more than an annoyance.
- Sixasis. Same as above, just complicated already established easier and more effective ways of doing things.
- Voice controls. Again sounds good and cool, but usually something you only really use to show off to your friends and never actually use as a day to day thing unless you are delibrately going outta your way to do so. In addition, everything that is done view voice controls, can usually always be easier to do with a simple button or an efficeint UI design. And more reliable. The second something takes longer to do than it would have another way, its redundant.
- WiiU gamepad. Honestly this one is up for debate. Does it or has it made any particluar game so much better that it redefines a genre? If its best feature is being able to extend your creen then sonys remote play has shown that there is a better more general way to do that. There is still time for nintendo to set this right though.