When prototyping a new game, you’ll have moments you think “what is this pile of garbage?” My advice: Walk it off and carry on anyway.
— Tory Niemann (@tkniemann)

Mark Major of Whirling Derby (@WhirlingDerby) offers advice for new designers:

“Every skill you have helps in game design, because everything can be the basis of experience for a game. Design is about creating an experience, so before anything else, know what that experience is and be realistic about whether you can deliver that or not. Don’t be afraid to cut material out of a game that doesn’t reinforce that experience.

"Also, spend a lot of time with your games. Make them shine. Playtest them. Making games and getting them out in front of people is exciting, but don’t let that excitement make you settle for second best. If you know you can do better, take the time to make it better.”

http://news.thegamecrafter.com/post/42445138546/mark-major-inducted-into-the-hall-of-fame

Learning to sort the essential from the non-essential is key. You can add all the bells and whistles you want to your design, but if it isn’t fun when boiled down to the core ideas, it isn’t going to be better just because you add more.
— Jason Kotarski (@jasonkotarski)
Make sure it’s fun. A designer can fall into a trap of working on something that the designer thinks is interesting, but that ends up not actually being fun.
— Michael Iachini (@OnlineDM1) of Clay Crucible Games
If you don’t have a passion for design, you’ll end up as a game idea generator rather than an actual game designer. You have to be able to actually build prototypes and test them and build new prototypes and test more.
— Michael Iachini (@OnlineDM1) of Clay Crucible Games