Your Game Will Probably Have a Client
In many instances, game developers have to please their clients - who are often the publishers or media companies that will distribute the game, or another company that is commissioning you to build a game for them.
The problem is that the client will often have very strong opinions about what the game ought to be, even if your ideas are different and your research suggests it is not in line with the desires of the present market. And more, they will want to try to control you to design the game according to their opinions.
If you allow the client to dictate the game design, you will end up with a bad game, and it will do your reputation harm. If you shoot down your client's suggestions, you may lose them as a client, which will do your bottom line harm.
The author suggests that the best way to deal with suggestions is to try to get to the reason why the client is making the suggestion - what they are trying to accomplish. Sometimes, the suggestion has nothing to do with what they really think is wrong (example: client wanted cars in a racing game to be red, because he felt the game was too slow and red cars would make it "feel" faster).
Another issue is that clients can be peevish and vague - the "bring me a rock" game (starts with a request, then dissatisfaction with the "rock" and every other "rock" the designer brings them). The task of the designer is to draw the details out of the client (what kind of rock? What are you going to do with it? Etc.) until you can figure out how to satisfy their need.
A lot of this seems very simplistic, and not at all unique to game design, but it's probably a good exploration for novices who haven't been through the wringer with clients.