Players/Consumers are a top priority in your game. After all, they ARE the ones you are designing it for, no? They could be doing a wide range of things for you; paying money to play your game, spending there time to download your product, making your dream job come true, etc. Without the consumer, there is no point in making your game, so, it is extremely important to keep them satisfied.

Making your level Unique
Why would a consumer play, or buy, the same game more than once? They obviously won't, and they probably don't like being told to buy it twice. The same goes for levels. The player wants the levels at least semi-unique. Try not to recycle graphics too many times. Even if the game is entirely based in a single building, you can at least have different carpets, enemies, objects, etc.
Of course, in games like Grand Theft Auto, or books like the Gor series, people will constantly buy the same game-- of course, with variations (at least slight,) in plot, setting, characters, art, etc. So, try to keep that in mind when you design your game.

Player’s don’t like playing, or indeed, buying, the same game twice. Of course, like Star Trek fans and readers of the prodigious Gor science fiction series, some players will continuously buy into the same formulae or even the same game with just slight variations in plot, setting, characters and art. The same can be said for level designs – people don’t like playing the same level twice. Not only does it ruin the entertainment value, it also fails to spark the imagination. It’s therefore incredibly important that levels introduce some variation in the plot, challenge, setting, and characters (i.e. the enemies.)

Challenging your Player
People buy games for a few reasons:: The challenge, the fun, and doing things they wouldn't do in real life. However, if you made a game with no challenge, it would take away from the fun. Then, once you take away from the fun, the game is screwed. Even if it is in a world inhabited by aliens that you can talk to. Shooters should test accuracy. Strategy games should test tactics. Sim games should test the players ability to think. That is something important to keep in mind.

Escaping Reality
As mentioned in "Challenging your Player," the player wants to do things they normally wouldn't do. They want to escape reality. They want to be in an alternate world. The setting needs to be designed in a way that it fits in with the world; It needs to convince them they aren't on Earth, sitting on there chair, playing the computer.
This one is commonly ignored, and shouldn't be: A mage shouldn't be telling you to press Ctrl+T to teleport. Leave that for the manual and settings screen. Try to keep everything in context. Nothing should be said that normally isn't said.
Think about the world the game is in while you code. Even if your game is based on Earth, the player should feel as if they are at that location.

Conclusion
This isn't a rulebook for level design. These are merely suggestions. However, they will greatly improve your game if you follow at least some of them. Remember, extremely high realism; boring. Extremely low; just creepy. Just try to keep these in mind while coding!~
Til next time, Happy designing!~