What makes a gimmick most obvious is when a sequel to an older game comes up with a new gimmick. This gives the game a feeling of newness. I'm sure you've all felt it at some point. You pick up the sequel to your favorite game and put in the disk and it doesn't feel like your favorite game. It's not always a good feeling. A perfect example is the Halo series.
The first Halo was a (now) standard FPS that switches to a third-person camera while in vehicles or on turrets. What made Halo special is that it increased realism (read, simplified controls) by limiting you to two weapons at a time. This was new at the time, and was done mostly to allow for workable shooter controls on a console, whereas most older shooters allowed the player to carry an entire arsenal, somehow.
Halo 2 added the idea of dual wielding certain weapons, which allowed players to customize their playing style more, while maintaining an easy control scheme by eliminating grenades when a second weapon was being used.
Halo 3 was the most gimmicky of the series, adding both heavy weapons and the generally next to useless “equipment.” And this is where I make my main point. Some games, like Halo 3, go out of their way to make space for their new gimmick. In this case, aside from the turrets that can be torn up from their mounts, constituting the main heavy weapons, Bungie came up with some other weapons that could be used the same way, similarly switching the camera to third-person. Why, you might ask, does a first-person shooter provide you with weapons that change the camera to third-person? Because that's the gimmick, and that's how it works. It's not necessarily bad. If you're like me and are easily entranced by the subtle, graceful curves of Sangheili Assault armor, it adds a little more enjoyability to playing online. Or MJOLNIR armor, whatever you're into. At the same time, however, it just doesn't quite feel like Halo.
One of the most recent examples is Gears of War 2. In this case, the makers must have been observing the players and thought, “This guy on the Troika is sitting there ripping all of his enemies to little pieces, but he's only using RT and the right stick. We need a new button.” And thus was born the idea of cooling rotary barreled weapons by pressing RB. This led to the creation of a whole line of air-cooled weaponry, including most vehicle mounted weapons, and the highly amusing “Mulcher.”
Not all gimmicks make this much sense, however. For instance, after the release of Star Wars: Battlefront, the designers must have realized that it was terribly irritating how slow the units are. So, in Battlefront 2 they added a sprint button, for everything, including vehicles. If a vehicle could sprint, it would move that fast all the time. They're machines, they don't get tired.
Some gimmicks stick, like the Quick Time Events popularized by Resident Evil 4 and God of War.
Some don't, like—hopefully—the ability to make an AT-ST sprint.
Really my point is that, a game and its influence can often be told best by which gimmicks they choose to rip off. For instance, when playing Perfect Dark Zero and Frontlines: Fuel of War, you can tell from the control layout and third-person vehicle system, respectively, that they are both taking inspiration from Halo. The main difference being that Halo was a good game.
That's pretty much all of have to say. Bear in mind that I'm not necessarily complaining about gimmicks. They add character to games, and account for a lot of the novelty of some new games. Plot and graphics are important aspects of course, but so is the feel, and the gimmicks make up a big part of the feel. Personally, I admit, I rather like Quick Time Events.
Except in Clive Barker's Jericho.
Jericho can fuck itself, seriously.