All my ratings are based on a mix of two things. How much I enjoy playing the game and how much the game means to me. I don't care about any other metrics (bugs, bad graphics, poor level design, sub-optimal controls, weak story, etc.) unless they impacted my enjoyment or ability to find meaning in the experience.

I only rate games I feel I've played enough to rate. For most games that's around 5-10 hours, but for simple games that's maybe a couple hours. For games that can be completed, I never rate it until I complete it. Additionally, I almost always stop playing games I don't enjoy after a few hours. Thus, the vast majority of my ratings will be 7 or higher.

I also include approximately how long I've played any given game as a note (in hours).

Ratings
10 - All-time favorite
9.5 - General favorite
9 - Love, nearly a favorite
8.5 - Love
8 - Like a lot
7.5 - Like
7 - Mostly like
6.5 - Like with reservations
6 - Ambivalent, some enjoyment
5.5 - Ambivalent, passes the time
5 - Mediocre, rather do something else

4, 3, 2, 1 - Never rated a game any of these. I'm pretty picky about which games I play, so I'm already very unlikely to play a game that I actively dislike. Take that and combine it with what I said before, that if I do rarely play a game I really don't like, I have always stopped before I played enough to consider rating it.