As a game designer, there’s nothing more valuable than notes on game ideas I’ve long since forgotten.
— J. Alex Kevern (@AlexWithIdeas)
Usually the quality of a game is a product of its execution, not any one idea, and it pays to get feedback from a diverse set of people.
— Matt Leacock (@mleacock)
Keeping in mind what makes your game different as you design the game will help you to highlight that element as you go through the design process. Think about the rest of your game acting as a funnel to bring your players back to the unique elements of your game.
— Chris Renshall

Cardboard Edison's Favorite Tips & Resources - October 2014

Playtest the hell out of your game. You’re never done playtesting it. Even when you think you’re done, playtest it more.
— Jon Gilmour (@jongilmour)
Do not be afraid to show your design to other designers. They have been where you are now and understand your feelings. It is a scary thing, but totally worth it.
— Suzanne Zinsli (@KabrtZinsli)