Every object, feature, story and mechanic in a game needs to have a purpose or a reason why it exists. Without a goal, a game will feel empty. Without a purpose, a character or item will be obsolete. Without a meaning, a story will have no impact.
Aim to give every aspect of your game a purpose, goal, meaning or reason.
Usefulness must be taken into account when designing a game or its content. Everything you put into a game should be useful to the player in some way. Or have a purpose to exist. If it doesn't have a purpose, the player will ignore it because its useless and there is no reason for it to exist.
There are games that attempt to create a world, a universe of their own that mimics the real world we live. It may try to recreate things like objects, physics, creatures, people, technology, environments, materials and other real world things to create a convincing representation of the real world, either as realistic as possible or altered with fiction and fantasy to any extent. Then there are other games that are much more abstract. These abstract games might have subtle references of the things mentioned above, but in their core, they define a completely different world of their own on which the game works.
A quick example to demonstrate the two types:
Real game = Minecraft, World of Warcraft, X-COM, Half-Life, EVE-Online, Red Alert.
Abstract game = Tetris, Pacman, Audiosurf, Breakout, Card Games, Pinball.
The distinction between the two is not always clear, although essentially if a game is based on something from the real world, rather than an artificial fabrication (that might use references from the real world), then its a real game. More accurately, a real-life inspired game.
Thing is, whatever gets put in the game must have a purpose. If you take a look at the nature, you'll see that every single thing has a function somewhere. There is absolutely nothing in the nature that has no point. Apply this same logic to a video game and it will become more natural, easier for a human to understand and interact with.
Obviously there games where you cannot be or have everything at once. You have to choose, usually due to limitations or restrictions. Be it a character class, abilities or equipment, my advice is to keep every choice a good one. So no matter what the choice is, it will always be the best, but just different in functionality. The player can pick anything he wants (or is able to) and not worry that the game might be unfair. He will then focus on having fun, rather than pondering if his choice was the best one the game can offer. Whatever the game offers, make every offer a best one.
Besides gameplay, a goal is extremely important in the overall story/plot of the game.
While you can make a sandbox game where people can do various activities, there needs to be a goal, an incentive to make their activities feel meaningful. If there is no point, the player will feel empty and there is no longer any satisfaction or fun had in the game. He won't play it anymore.
The above is often the fall of many sandbox games that allow you to do and be anything you want, but doesn't give you a constant, meaningful goal to achieve. This is even the more critical if the actual gameplay mechanics are limited or poorly designed.
Same thing with life itself. What is the point in life? People often have an activity they enjoy doing or a goal towards they work for that has value to them. You can create this enjoyment and value for video games too.
When you give your game a point that actually matters, something the player could care about personally, you can potentially create a game that is more important than life itself.
It can be scary how big the potential is, but it is true and some popular games have actually achieved this. With purpose, your game could be virtually infinite and endless.
So, in short, make sure that whatever is in a game has a point, a use or a reason to exist. If it doesn't, either make it have one or remove it. Focus on what is necessary to enjoy your game and keep away the redundant things that have no meaning.
(This is a part of the "what makes a good game" series)