You cannot skimp in any department. Theme needs to be fresh, art needs to be top notch, and gameplay has to add something new! Just do not be lazy in any aspect of the game development.
— Dan Letzring
Make sure you look critically at every aspect of your game, and have new players look at your game with fresh eyes. They may notice something you’ve missed.
— Chris Anderson
Drown your game in passion and play testing. Get hundreds of people to play it, and then listen to their feedback. Specifically, listen to the problems they isolate, but not necessarily their solutions. That’s your job.
— Marc Neidlinger