I think you are all right here. In my opinion you indeed need a prototype phase with a strong focus on gameplay. But at the same time you should have a clear vision of style and mood you want to create. Arts, music and story are very important to achieve all this. In the end you will fail if only one of these components are too bad to please your potential customer.

And that is the reason why small indies have small projects: The successful ones make good games with good gameplay, good graphics, good music and an interesting story. And to finish this in time they choose something smaller. All the successful stories I have read about started quite small with casual games, flash games, web or mobile games, adventures or simulations. After that they grow bigger.

And they all tried to balance all components of their products, not only game-play. It would not work at all, just take Adventures as an example. The gameplay is not new at all, the story, the art and the mood will immerse the player.

A gamer expects that gameplay just works, that there are incentives to go on, that there is something to explore, something interesting to reveal. I cant remember any gamer to say: "Hey, the controls were amazing and the camera view was so great!"
While this is really very important especially in a prototype phase, but the gamer wants that it just works and will probably not mention it as a feature of your game, except it is really very exotic.