The story! A good story encompasses well written characters, a driven plot, and good pacing. Whether it's a comic, novel, or movie, if the story is poor, nothing else can save it. In the case of a movie, you can have a great cast of actors, amazing set design, but the script can be garbage. The actors can try their best with their delivery, but they can't save the movie from flopping at the box office. It'll just be 'glitz and glamour' without the heart and depth a story should have. An example would be the butchered script of Justice League versus the actual vision of Zack Snyder's Justice League.
With a comic, the artwork doesn't have to be stellar if the story really shines through. I've heard a lot of good things about One Punch Man and from what I understand, the creator wasn't exactly the greatest artist, but people were so invested with how great the story was. When a story can pull you in like that, you can look past the artwork.