Basically, if I give something below a 7, I regret watching it. A 7 is neutral, and anything above that means it was worth the experience, and I gained something from it. I also place heavy emphasis on enjoyment when I rate something, so my ratings are somewhat biased. But aren't everyone's? :D For me, enjoyment mostly consists of character development and plot twists (or at least how exciting it is). That's why I tend to go easy on funny and light-hearted anime -- I'm harsher on dark/serious shows that thrive off of character development but may come off as somewhat dull. The second most important thing for me is sound (voice acting, OST, etc.), and then art. If an anime has really good sound and/or art, it's going to make a huge impression on me and can really boost its score -- for example, Fairy Tail has amazing music. However, those are just bonuses, and can't make up for bad character development and a boring story (to clarify, I don't really care about the plot, only if it's executed well). On the other hand, I can deal with bad sound and art if everything else is good, but it will seriously detract from the score -- a prime example is Elfen Lied.
0th anime: The Secret World of Arrietty
(anime before I knew what anime was xD)
1st anime: School Rumble
25th anime: Elfen Lied
50th anime: Densetsu no Yuusha no Densetsu: Iris Report
75th anime: Free! Eternal Summer
100th anime: Death Billiards
125th anime: Sore ga Seiyuu!
150th anime: Kimi no Na wa
175th anime: Getting there!
I don't read manga that often, but I'm currently desperately trying to catch up to One Piece.