There is also a who crapload of game design theory to be learned as well, though there aren't many sources to learn it from. Having consulted for a game development company and being a veteran gamer with hundreds of different systems under my belt, I have a pretty good feel for ideas that will work and those that won't when it comes to the mechanics of a game. You can have a fantastic idea for a game with a great development team, and still make a poor game if you don't get the mechanics right.
Good examples are: Sacred (good story, good dev team, terribly clunky mechanics), Master of Orion III (successor to one of the most popular strategy franchises ever, great dev team, but the game's mechanics were wholly dissociated with the rest of the game - like playing chess with Monopoly rules tacked on), Axis and Allies (great concept, poorly executed mechanics)