This got me thinking a bit. Let's see...my top 3s.
1) Story - To me, the story is the single most important aspect of a book, comic, movie, etc. There needs to be good character development. I like good dialogues. The story has to sway my emotions, make me laugh, sad, angry, scared, etc. I prefer something that's controversial than something boring, but I hate anything distastefully done like offending someone without reason (trust me, I'm not easily offended like some peeps today).
2) Art flow (aka Storytelling) - Story can be good, but how your tell it can make it great! This is the power of comics vs. a book. Book is great because I like to use my imagination. Comics is great because the creator invites me to his or her mind. Art flow can make something as simple as walking in to park feel exciting.
At Tapastic, we keep reiterating the power of digital / vertical formats / mobile optimization and there is a reason for it. We allow infinite vertical scrolling, gifs, soundcloud, and more stuff will come. Free your imagination.
For example, I am absolutely in love with Juju's Diary.
3) Being honest with you story and art - I personally like comics that reflects the personality of the creator. It's hard to describe, but when I read the comic, I feel like I am also getting to learn who the creator is.
For example, I feel like Fail By Error by @Min is as honest as it can be. I also fell in love with Girls in real life by Moneydog recently. Yup, and of course, @feyuca's Two Cent Slice is so much fun.