First and foremost the writing -- the story needs to be interesting and the protagonist characters need to be likeable or relateable so that I care about the characters and what happens to them, or the comic won't keep my interest at all, but the artwork also needs to be of decent quality or at least aesthetically pleasing to make reading the comic a pleasant experience. After all, it's the visual illustration of comics that differentiates them from purely written stories, and good artwork is the first thing that draws the eye and makes people notice a comic, along with unique or aesthetically pleasing drawing style and character design.
Comics are a storytelling medium (most of the time), so the writing is paramount. I still enjoyed V for Vendetta or Watchmen even though I think their art is mediocre. They're LESSER comics for having lesser art; it's a VISUAL storytelling medium, so it should utilize both well. But I'll pretty much always enjoy a comic more if it's written well than if it's drawn well if it can only be one of the two.
The comics Iown have a good flow to their design and aren't messy in presentation. Most of my stuff is late seventies or younger. As a person with dyslexia, it can sometimes be hard to read and follow the newer comics because of their style (my friend has four Batman & Robin comics and I have no idea what happens in them beyond the pictures). I am particular about what I buy because of this.
The characters are usually the reason I picked up the book in the first place. After reading the first thing I get hooked on is the art if I like the style. Some books I'll get just because it's drawn by a certain artist. Story is also important but sometimes it's hard to get through a comic with a great story is the art isn't appealing unless it's a story with characters I already love.
It's usually the story line that captivates and makes me keep reading and if the art isn't all to great, I can keep reading. But if the art is really good then I might end up getting lost in it instead of the story which is fine by me because then I get to reread
I adore David Mack's Kabuki series, the art is so wonderful and he got a really amazing team of artists to work on follow up stories with him, I really wish he'd finished them all instead of running off to do daredevil. <3 I have to say art above everything else because that is what drew me to his work, he plays with so many styles, bleeding watercolors and overlays and surrealism, each page is a treat!