How I rate my entries (which can be seen in the tag section):
0-5 rate for art, which includes animation, character designs, background art and visual delightful. (A on my tags)
0-5 rate for story and character development. (S on my tags)
0-5 rate for audio, which includes voice acting, opening/ending and insert songs, background music, sound edition, voice acting and how well does it all blends with the visuals displayed. (M on my tags)
0-5 rate for how much I was entertained by the Anime, completely subjective. (E on my tags)
I assign 2.5 points per each category based on the rate and sum it up to get the total score for the Anime. If the sum ends up with decimals, I tend to round up or down depending of how ground breaking I think it is or how much I like it. I also tend to consider each Anime type different. So a Blu-Ray bonus material is rated based on how well does it perform as a bonus material, not comparing it to a TV Anime or a Movie.
I usually don't drop a series and try to finish it once I started watching it, in order to have a more educated opinion about the show as a whole. But sometimes I just need a time off from a series I'm not enjoying that much, which is why I have so many lurking Anime on my "On Hold" tab. I hope to finish them all someday.