Think of your favorite games. Why did you enjoy playing them? Probably because they piqued your curiosity and gave you a feeling of success. Why did you stop playing others? Because they were boring or frustrating.
Unsurprisingly, criteria such as "special effects not colorful enough" or "cartoon characters not crazy enough" rank quite low among the reasons why not to play a game. These factors are the least of your worries when designing a fun game. But how do you find out whether your game idea is "fun"? Let's see what the experts say. Here is a quote by Raph Koster from "A Theory of Fun for Game Design":
"A fun game is the one that offers an interesting challenge to the player and makes him want to play the game until the challenge is conquered. But, the trick is to make the challenge not too hard (it will cause frustration) or too easy (it will cause boredom), so that the player can keep on playing."
A challenge becomes interesting if players feel they have a choice and control over the outcome. If the challenge is "not too hard, nor too easy", it is balanced. What we mean by interesting and balanced will become quite clear when you will look at a few examples.
Put your game idea to the test and answer the following questions:
Does your game have an item such as a "+50 Bazooka of Deadly Deadliness", a weapon with no disadvantages and which is so powerful that there is no reason to own any other? If yes, then you have deprived players of the fun of choosing the optimal equipment.
Does your game exclusively generate enemies that are of equal strength as the player? Then you have deprived your players of the fun of taking smart risks. Make sure your game does not encourage arbitrary behavior—in this case, "I attack whomever I meet with no consequences."
Does your game have lots of weak items that cannot be sold or used, and whose only purpose is to add volume to a pile of loot? If yes, you have deprived your players of the fun of identifying valuable items.
You see that challenges are only meaningful if players face choices that noticeably influence the outcome of the challenge. How does your game idea fare regarding these important criteria?