1) Design and overall appeal to the reader
2) Storyline character development with the plot and protagonists
3) ARtwork and scenery and how well they are developed in relatrionship to the story line "this also applies in making wallpapers"
4) Its "catch appeal <--what i like to call it" It's that something that can drawl a reader or watcher in.
5) Popularity -> lets face it some people only watch a anime cause other people they know do.