You can waste all the time you like making rules that work perfectly, and mechanics that are new and slick, but in the end will people really want to play your game again?
What are the game elements that draw you into games and keep you coming back for more? For me, I enjoy building things, and the satisfaction that comes from believing that my creation was better than your creation. This is what drove me to love Battletech so much - even though you were limited in how you could build your mechs, it still felt like you were optimizing them for the type of combat you wanted to engage in. I also like Magic: The Gathering almost completely because of deckbuilding. I like to discover synergies between cards, and put together a deck that can handle most obstacles.
So, in your opinion, what makes a game fun -or possibly more important, not fun?