Regarding school grades, I'm in the UK and I can't remember school (you leave at 16, I'm now 26. Wow 10 years) but at university over here 40% is a pass, 50% is a 2.2, 60% is a 2.1 and 70% is a first. And the guy who made the points about 50% not meaning you only know half of the course is dead right. Also, I understand many American tests are multiple choice, in which case there should be a very high pass mark, our tests generally aren't like that.
Anyway on topic, PC Gamer (UK) used to run a sensible scoring system, 70% was a good game, 80%+ varying degrees of greatness. In fact they re-released some old games as part of a special 80%+ series. Down to about 60% were recommended if you were a particular fan of the series or the genre.
It worked and it was sensible. I'm a big fan of percentage systems as they're more sensible than any 5/10/11/12/whatever point system. Perfection would be 100%, any less is well, less.
The big advantage of the times when they used to score games properly was that you had more differentiation. GTA IV, COD MW2 etc all get perfect (or near as damn it) scores, what then happens if Black Ops is better?