I like to follow the anime list way of rating, and that system goes as follows:
10 - masterpiece
9 - great
8 - very good
7 - good
6 - fine
5 - average
4 - bad
3 - very bad
2 - horrible
1 - unwatchable
When grading, I break the show into several sections and grade each of those.
Plot- Obviously, plot is an important part of the show. How they twined the events together; how everything was presented; the level of suspense they were able to work into the show: these are all things I look at while grading plot. (I really don't think so deeply about it, though. I just kind of sit there and wonder 'is the plot of this show worthy of this grade?')
Music- How the music (both BGM, OP, and ED) affected the show. If they used the proper BGM to make scenes better.
Ending- A show can have a terrible plot but a fantastic ending (I'm looking at you- Code Geass R2), so I find it important to grade this separately. Did it leave a good or bad aftertaste? Did it tie all the lose ends together?
Art- This also covers animation, but I'm too lazy to write out "Art & Animation" every time. Was there a lot of derp faces? Was the art and animation consistent? Was it unique? How detailed were the backgrounds? Was the animation smooth without sudden stills?
Characters- Were they annoying? Were they inspiring? Were their back stories developed? Were there unnecessary characters?
Personal Enjoyment- A show can be terrible, but still a very enjoyable experience. Was I entertained throughout the episodes? Was I ever bored? Was I in a rush to get it done?