So you have fallen in love with a great idea for a game and have begun to brainstorm 1001 ideas of mechanics you'd like to see in it. You may feel inclined to give the player access to 35 weapons, 40 super powers, and a plethora of cool gadgets the player can utilize in order to defeat their enemies and slay the final boss. While this may sound "really cool" and interesting the fact of the matter is that it actually just waters down your game. A well thought out and fully explored mechanic with multiple uses will always be more engaging to the player than simply piling on lots of different ones. A less is more philosophy should be constantly in the back of a good developer's mind when designing a game. If you find a mechanic in your game that has only one use then examine that mechanic and see if you can "juice it" to create more depth, if you cannot then you may want to consider cutting it.
Example 1: Legend of Zelda - A Link to the Past Edit
In the Legend of Zelda - A Link to the Past, almost all of the items you receive in the game have multiple uses. Bombs can clear pathways and kill enemies. Jars can hold a variety of different items. However the boomerang is one of the better examples due to its versatility. It can be used to kill smaller enemies as well as to stun larger enemies ones and can also be used to pull items towards you that are out of your reach. There are even puzzles which require you to hit switches from far distances it on order to proceed, if only you had something you could throw at them and then get it back....
Example 2: Wolfenstein: The New Order (2014, Bethesda Softworks) Edit
The New Order, takes place in an alternate universe where the entire world is controlled under the Nazi's. It offers a wide variety of combat experiences. There are various types of guns, and some guns can be upgraded to fire bouncing bullets (like in Unreal Tournament). However the laser wielding gun is one of the better examples probably due to its versatile use. It can shoot enemy robots, cut through metal sheets to explore hidden pathways. The gun needs to have a said amount of charge to use it. The game offers a multitude of charging stations for the player to use. Also, the gun has a fixed range, so the player needs to get up close to the enemy. With an excessive amount of guns and great storytelling, Wolfenstein's mechanics stay to the core as a first person shooter.