I tend to be pretty ruthless when it comes to ratings. We're given the scale of 1 to 10 after all, so I use the full spectrum. Anything 7 and above can be considered very good.
My Rating System
10- perfection, or as close as it can get can get!
9- consistently good. So it is only different from an 8 because it doesn't have much noticeable flaws, and different from a 10 because something very minor just doesn't agree with me
8- can already be considered as one of my favourite shows, but probably has a few flaws which bring it down a notch
6- worth my time (naturally, shorter shows will get this score more easily)
5- if I had known better, I wouldn't have completed it
4- any time spent after the watching first few episodes is time wasted
3- complete waste of my time
1- one of the great regrets of my life