I figured I might as well explain what I have in mind when I grade something since everybody has different methods of how they determine what is a #/10.
First off, my grading scale is pretty much aligned with the American schools' grading scale:
10/10 = A+/S
9/10 = A
8/10 = B
7/10 = C
6/10 = D
5/10 and lower = F
On a basic level this is how I view things when I grade them: I see everything in letter grades.
Now to dive in a little deeper, If I had to give one word or phrase to describe each #/10 here's what it'd be:
10/10 - Masterpiece
I only dish these out if the anime/manga impacted deeply on multiple levels. Obviously, there's no such thing as a "real" masterpiece, but these are the titles that truly wowed me and I believe are perfect or damn near perfect, even with their flaws.
Now I consider everything below a 6/10 a F, but there's different levels of how bad it failed:
5/10 - Mediocre
Basically, there's no real qualities of the title that makes it stand out, not even a little bit. While a 6 has at least a few things that I thought were decent, a 5 has close to none. It just goes through the motions, but it isn't necessarily terrible either. It's just an average joe anime with nothing really special about it. I find that worthy of an F.
4/10 - Bad
3/10 - Horrible
This is usually the lowest I'll go for a F.
2/10 - Agonizing
It was so bad, that it caused real physical and mental pain. Why did I put myself through this!?
1/10 - Garbage
These shows I don't even hate, I can't even give them that luxury. They are akin to trash and should be treated as such. Do you think much about that paper cup or the gum wrapper that you threw away recently? I didn't think so.
Usually when grading a show, I have three methods of grading. Usually I use all three when grading a show, but it depends on the show so I may critique the show differently by favoring one method over another or sometimes using all three equally, etc. Here's the methods:
1. I'll compare it to titles within its genre. How does the show stack up against other shows in its genre(s)? Does it do things differently? Does it do things the same? How does it use these devices?
2. I'll think how it compares to other titles in general (so outside its genre). Are the themes, narrative, characters, animation, art, atmosphere, tone, music, etc. good in a general sense?
3. How did it affect me on a personal level? And was I entertained when watching/reading the title?
That is how I usually think out things when I grade them.