for most games, having decent graphics, music, good sound and great controls are kinda essential to have any fun at allWell, to some people like me, those stuffs you've mentioned except for great controls are NOT essential to have fun.
The simple reason prototypes don't need those stuffs is they need to be made quick. Omitting those stuffs is one best way to do it.
Also have you played any game with decent graphics, music, good sound and great controls but horrible gameplay?
if you are for example developing a jump & run or a regular RTS, do you really need to test if the idea is fun? We already know these genres are fun
Sure, those genres are fun but the real question is : is your take of the genre fun? Ex: if you were to make an RTS, are you sure it would be fun?
Back to my point above, aside of being quick, removing graphics, music and sounds would make it easier to focus on the gameplay. There are many aspects to set up properly to make it fun.