I might add that I'm only really discussing his impact on their musical output as a finished product, though. A band is much more than the recordings it releases - it's a functioning three-way, four-way or more-way marriage between complicated people. Anyone who thinks being in a band is easy is smoking something. It'd be impossible for outsiders to truly measure how Queen would be different as a band on the whole because none of us was there. That's not even taking into account his involvement in their business affairs and all of that less-interesting crap.
But yeah. If we're just talking the finished musical output, then John's final, superficial contribution is a smaller one, and would make less of a difference if it was suddenly gone than say, Brian or Freddie's.
john bodega