Legibility is a big'un for me. Like... if the comic is physically difficult to read (be it due to font choice, font size, or other factors like hard to read colors, poor contrast, transparent speech bubbles, etc.) then I wont bother with it.
"Bad" art has to be really awful before it's a big turn off, but a bad story is far worse. If I hate the story it doesn't matter how pretty the art is, I'm not reading it.
I guess those are the only two universal things for me. Everything else is highly subjective. But legibility and being interesting are non-negotiable xD