The problem as I understand it is that the "rating space" is multidimensional (a piece can be funny with great plot but bad writing compensated for by a great lead and awesome special effects although the set design was inconsistent) but all of this has to be projected into a 1-dimensional system.

And the rub is that not everybody uses the same projection, which means the utility of one person's rating for another unknown their projection system (the weight they give to each sub-component of the overall rating) is completely unknown.