So part of being a game designer is actually creating meaningful content for games and judging whether or not that content actually works for the overall structure of the game.  Most people know about the first one (though they think it's just creating "ideas for games" which is just wrong) and though the second seems easy enough to grasp it's certainly more difficult than it seems.  The problem with the "judging content" part actually comes from the "creation" part, in that people get attached to what they create.  You can see this in more places than game design quite frankly, because it will stem from anywhere a person has a personal stake in the creation.  Once you've created something you're happy with it can be difficult to let that thing go and in some cases I've seen game designers then try to restructure an entire game around this one concept they created.  Generally speaking this completely ruins the game and even though the designer might be satisfied that their creation made it into the game, the experience as a whole has been destroyed.

I found an interesting article about a game called Amnesia, which is one of the best horror games I've ever had the opportunity to play.  But in the article they describe how they started off by building the game around a mechanic that eventually in practice didn't really work as well as they hoped.  Eventually the mechanic that they once used to help build the game was reworked and made a much smaller part of the game, but overall the whole experience was dramatically better for it.

Article:  http://www.gamasutra.com/view/feature/223931/game_design_deep_dive_amnesias_.php