Consistency is vital. Its extremely important. Without it, your game has a large risk of becoming a horrible mess with the effect of the player not trusting the game because he notices that it cheats on him or does something completely illogical or just plain ridiculous that makes him raise his arms and shout "bullshit!".
This eventually makes him stop playing the game altogether because no one likes it when they're being lied to. And this is what consistency is all about: Trust.
Good consistency ensures the trust of the player. When everything functions as it logically should, the player can safely have pure fun without thinking what stupid crap the game will pull off next. Sure, there are games that do this and even as far as focus on this randomness, but these are rarely those serious games that have endless potential and infinite gameplay. Such games are very shortlived anyway.
What is consistency?
Its about organizing the game and its mechanics that it all works seamlessly together as logically as possible. This makes the game feels natural.
Look at it this way: If you do X, Y will happen. If you jump, you will fly up into the air. If there is gravity, you will fall back down. You set fire to a piece of wood, it burns. You burn ice, it melts. You burn metal, it will get hot, but it wont catch on fire. A ball hits a wall, it collides with it.
Its all about simple logical thinking, basic interaction and feedback of elements. This is what good consistency is all about.
If you give a player a character or object to control or interact with, make sure it has all the features you would expect from it logically. Atleast the very basics and don't limit it with cheap limitations or other illogical restrictions.
Consistency is important in everywhere. Both in story, gameplay, behaviour of objects, potential of what characters are capable of doing, environments, difficulty and so on.
What is inconsistency then?
Example: You control a human. It makes sense that you can do all the following:
Walk, Run, Jump, Crouch, Climb, Look, Smell, Eat, Hear and Interact with solid objects using your body. Its all part of being a human, after all.
It doesn't make sense if you lack all or some these features. Wouldn't it feel strange if there was small rock in your way and you can't jump on it or climb over it? Or if there was a ladder or a door that you can't climb or open?
That is not consistent. You were given a human to control, as the game claims, so its obvious you should be able to do those actions, but if you can't, then the game is fraud and unfair. You're being lied to.
This logic applies to everything, not just this example of a human. Be it a world, story, a mechanic or behaviour of objects and characters. It all matters how it behaves. When something doesn't work as it would by logic, the player will becomed distanced from the game, the immersion breaks and the player can't properly enjoy the game as he can't trust it. While the player might try enjoy the other parts of the game, the damage has already been done.
So get it done right from the beginning and avoid ruining the game with inconsistency by organizing the system and its element in a consistent manner.
Spawning and origin.
One very infamous element of bad design is spawning things. If you've played games where you have to fight enemies of any kind, you might remember that there are instances where monsters are spawned out of thin air, teleported in your face or appear suddenly behind the corner. This is even worse when the enemies are endless and spawn all the time and you can't do anything about it. This is probably one of the worst inconsistencies you can ever have in any game. Especially if its based on real-life, even partially.
Not only its unfair, but its ridiculous and illogical as well.
Everything has a beginning. Everything has an origin from where it comes. This applies to every part of the game too. World, characters, items, monsters, story... all of it and more have an origin, a beginning.
Always think where something comes from before you put it there and make sure the player will understand this origin of whatever comes up in his way during the game too.
Also, If you really have to spawn something out of thin air, at the very least, apply a limit to how many can spawn and inform the player about it, or he will get frustrated. Always be fair to the player, keep his trust up and never lie to him.
Player trust and consistency is key to a good game.
(This is a part of the "what makes a good game" series)