So you want to be a designer? Me too. When I ask designers what skills they need to do their jobs, overwhelmingly the thing they say is most important is being able to communicate with the team. This is fairly straightforward, since designers need to be able to share their ideas with both programmers and artists.
What they don't say is the secret deep down reason designers need to be able to communicate. I have heard it said that when a game fails it is the designer's fault. Or at least, everyone blames the designer. Why? Because the primary determining factor of a game's success is if people want to play it, which usually translates to if the game is fun.
Fun is the designer's job. But the designer can't make the game fun, not alone. As the designer, you may have fantastic ideas for the most fun game ever, but you need an artist to create the world and you need a programmer to make it function. So if you tell them the idea and they don't get it, then all that "fun" in your head isn't going to be in the game. What's more - during development as the designer you will probably be playing the game as it progresses, and you may notice problems with the functionality or the art, things that are stopping the fun, or you may have ideas for how the experience could be better. And if you don't say these things well, or worse if you don't say them at all, then the game progress will stagnate, it will not improve.
Communication doesn't just mean talking. Communicating with your team means getting ideas out of YOUR head and into other people's heads. You can do this by talking. You can do this with spreadsheets. You can do this with pictures. You can do this with paper prototypes. Communicating also means listening, because you won't always be the one with the good ideas. Always listen for the good ideas, because they come from every member on the team, and as the designer you have the power to take those good ideas and make them a reality. If the game ends up fun and you didn't communicate with the team, then you weren't really the designer.