I' m kinda torn with the scale issue come to think of it. I tend to think of games that rate in at a 5 or 6 as playable, but not enjoyable. A chore.
Superman Returns for example, was a 5 for me, maybe a 6. Playable, nothing's really broken, but like working the buttons on your VCR remote, it's not fun to play with. You don't feel really feel challenged or engaged for the most part.
If I see a 7, I tend to take that as the game is playable, and you will enjoy playing it. In this day and age against current games, I'd rate Super Mario Bro classic a 7. Everything works, no overly game breaking bugs, no story there, but it is fun to play and makes up for it all in the category of enjoyment.
But overall, it's a pretty hard and subjective thing to work out.