A good board game challenges you to make decisions. A 4 year old may not have the mental strength to know which option to choose, or even be aware that one option may be better than another, but allowing them to explore this decision making process helps them learn the closest thing we have to a fundamental truth, cause and effect.
A good game should present interesting, definitive choices. Should you play it safe or take a risk? Do you defer short term gain for a big payoff later on? Save your strength or make your move? Don't make the choices arbitrary.
A good game is good all the way through. It should be fun at the start, during the middle, and at the end.
A good game doesn't need its theme. If you numbered chess pieces instead of using a royal court, it would hardly lose its appeal. Turn Candyland into VegetableLand and kids will run away screaming from Lord Legume.
Not sure what this pic of Owen tied to a rocket has to do with the game theme, but it's still awesome.