Making the Game Fun
The last part of designing a game is the reality check. Do you really think that the game will be fun and people will like it? Or are you lying to yourself? This is a serious question. There are about 10,000 games on the shelves and about 9,900 companies going out of business, so think about this. If you're totally excited about the game and can imagine wanting to play it more than anything, you're in the ballpark. But if you, as the designer, just get lukewarm about the idea, imagine how other people are going to feel about it!
The key here is to do a lot of thinking and beta testing of the game and add all kinds of cool features, because in the end it's the details that make a game fun. It's like fine workmanship on a piece of handcrafted oak furniture (I personally hate wood, but work with me). People appreciate the details.