And unless we judge about the physical qualities of a product, all other aspects are simply a matter of tastes.
All other aspects? Hardly. It would, however, take a lot of work to properly judge many other aspects.
For example, there are mathematical measures that could be applied to RPGs to measure their complexity. We could then do studies over wide audiences to examine the correlation between that complexity, and various aspects of play experience. We might then might actually judge whether a game was "too complicated".