I'm on record, in my book and elsewhere, as not thinking very much of rating systems. Their basic flaw is that they give a sheen of objectivity to something that is inherently subjective.
I had a long conversation about this with Jim Murray once and his conclusion was, "we owe it to people to give them some kind of guidance."
That point I concede. Therefore, I'm comfortable with a four or five point scale along these lines:
0 = bad
1 = barely acceptable
2 = good
3 = very good
4 = great
My problem with the 100 point systems is exactly what comes up here from time to time. "How can X get a 96 when Y gets a 95?" Consequently, one of the rules of my proposed system is no fractions, no 3 1/2 stars. The idea is to rate in a broad way without ranking.
Subjectivity is still an issue as it always will be. The best bet for someone who wants to use anyone's ratings for guidance is to get to know the tastes of different critics. Either find one who generally agrees with you or just evaluate all of them based on what they seem to like and dislike.
Anyway, I don't want to write a long essay here. I just mean to open the discussion.