To aid with assigning (and understanding) anime ratings on MAL and anidb, I wrote the following cheatsheet. It's tailored for me, but you can make a custom one – the point is to describe your mental rules in detail. Try it, this makes grading easier.
Top to bottom:
Epic
Series which feel awesome and "solid" in some way: have serious, immersive storyline, quality plot, deep character development, reliably strong humor (e.g. not just puns and cutesy), balanced scene selection/pace/composition. To be "touching" is usually not enough: good "emotional" series rely on something factual (solid plot/character development).
| 10 | Life changing (on a personal level). There can be only two or three of such. |
| 9 | Anime of the generation. Can rewatch again and again (maybe in several years). |
| 8 | Awesome. Usually has two or more "professional" qualities. After watching you think you may want to rewatch it some day. If someone asks me whether they should watch this: "Absolutely". |
Good
Series which while not being "awesome" were pretty good. They key to the inclusion is that I honestly liked the series, not just watched it out of duty or being bored.
| 7 | Reliably good. Animes which have no "slightly underachieving" feel at all, but are still not "awesome and made perfectly just for me". You wouldn't usually rewatch such an anime, but you would actively decide to watch it if you haven't yet. If someone asks me whether they should watch this: "Yeah". |
| 6 | Slightly underachieving. It's not that it wasn't good, it was. I don't regret watching it. But I wouldn't regret missing it either. Most good "cutesy" anime, daily life anime without special traits (strong humor, chardev/drama) fall under this. Most good "short term shounen" adventures with peculiar worlds/contexts but classic themes of teamship/friendship, grownups, responsibility etc, and without other redeeming traits fall here or under average/ok. If someone asks me whether they should watch this: "If you want". |
Average
To get in this league the series has to not be anything from the Bad league while not being good. I cannot say I honestly liked this series.
| 5 | Average though ok. I wasn't much bored and I had some enjoyment while watching this (I might even call it "funny"), but if someone asks me whether they should watch this, I would reply "probably not". |
| 4 | Average and boring. Not really bad or *that* cliched, but I was bored and probably dropped the series. |
Bad
The series is extremely cliched, boring, it appeals to values I don't approve of, and not in a talented way. Most talentless ecchi without redeeming qualities (strong humor, originality) falls under this. Stupid/trash gag/toilet humor comedies. Hopelessly cliched kid/first time viewer oriented shounen shows. Series with a deep philosophical pretense and the lack thereof.
| 3 | Simply awful. Most utterly trash series go here. They wasted my time and I hate what I've seen, but I wouldn't care if I haven't just watched that. |
| 2 | Active repulsion. The series angers me. I'm against it or what it's trying to convey or the way it tries to act on a viewer (e.g. by appealing to his unsightly instincts or changing him for the worse). |
| 1 | I hate this with a fury of thousand suns. This series should not exist. If only I could, I would tear it from the reality and make everyone forget it and I would feel so much better. |