Animation quality. Series with low budget animation do not catch my interest.
The first fight (that lasted for about ten seconds) in Soul Eater was well animated and that's what ultimately caught my interest, but then the rest of the fights weren't as interesting, but the characters made me want to watch the rest.
And also character design and character personality. Like Maka Albarn from Soul Eater. The first thing that caught my eye were her awesome boots, and then next is that she's pretty badass, so then I liked Soul Eater even more.