In the very moment I'm writing this, I'm drinking a Moscow Mule. It's a cocktail, combining Ginger Beer, Vodka, and Lime Juice for a delicious buzz of spice and citrus. One of its biggest characteristics however isn't the mixture, but what it's served in.
Traditionally, the drink is served in an all Copper Mug.
Why?
...
Uh.
It gets colder quicker than an ordinary glass?
Well, that's par for the course. Any cold drink is better the colder it is (as long as its not ice). So while that part about the copper mug might be true, it ain't special to the Moscow Mule.
That makes this a gimmick.
Thing is... I'm still drinking it, and loving it.
Gimmick is used as a very negative word in gaming circles and understandably so. When you think "gimmick" things like the Xbox Kinect, Playstation Eyetoy, or the Dog from Call of Duty Ghosts.
Hardly enticing concepts.
However, the word itself doesn't have any negative or even positive connotations. It just means "augment with an extra device or feature." That's a broad brush. That covers everything from Guitar Hero to Max Payne. Paladins to Fallout. iRacing to Mario Maker.
Frankly... I miss gimmicks.
I miss when a game could be characterized by a single mechanic that made it stand out from the crowd. Battlefield 2: Modern Combat's hotswapping feature was such a notable "gimmick" that I still remember it within a fairly average 2000's shooter. Max Payne pushed an entire sub-genre of third-person shooters with its bullet-time mechanic. And I don't need to delve into Guitar Hero's revolution of the rhythm genre.
Really, it's not that "Gimmick" is an negative. It's all about the quality of the gimmick. Just like you'd judge Multiplayer, New Game+, or Character Builds, notable gimmicks are no different. They are apart of the game, and it's all about if they add or detract from the experience.
A tacked on gimmick can really dampen the players experience, but a great one, can make it stand the test of time.