The Look and Feel of a World is Defined by its Aesthetics
The aesthetics of a game are too often dismissed as inconsequential - but often, "good art" has a lot to do with the perception of the quality of the game.
He also advises against brining in the artists after the game has been designed. They have good ideas, and often their illustrations can inspire others and improve the design of the game as a whole.
Also, a game designer is not necessarily an artist - the two are different skills - and as a game designer, you can interfere with the artistic outcome if you are too detailed in your illustrations.
Therefore, sketch rather than design: your illustrations should be bare-bones and purpose-driven, to visually communicate ideas, not depict the appearance of the game. Leave that to those with artistic talent.
And a s a final note in a chapter of random stuff: when you hire artists, hire artists who are accomplished in the medium and know its capabilities. Especially when it comes to video games, artists who are unfamiliar with the medium can wreak havoc.