Game Ratings are fun and a little tough to do.  If you want to actually do a fair job.  I mean really, what truly costitutes the differences between platforms?  Some of the major things to look at are pretty simple: Graphics, Frame Rate and Features.  What makes it difficult is when you take a game like Rainbow Six 3.  When rating for the Xbox it is simple, it has a ton of features.  However the title doesn't have the same feautures when played on the GCN or the PS2.  Online is missing in a big way.  That is a great and expected feature on Xbox, however GCN and PS2 are not in the same league.  That is why it is hard. 

How to judge fairly on the game for the system it is being played on.  I do believe that some Gamespot reviewers forget about this little nuance when they were rating last generation games, especially the Clancy titles.