In a recent superhero comic, the artist introduced a character who was notable for his small stature. Nowhere in the first 20-odd-page issue did you see him clearly in scale next to normal-sized characters that demonstrated he was small. This was covered in passing in the dialogue. That’s bad comics. If you were telling this story over a campfire, one of the first things you’d say is that this guy was very short; you might make it specific, with a comparison. In comics, the art should do that for you. Something about a picture painting a thousand words. Visual info should be conveyed visually.
-- Dark Horse editor Scott Allie, not referring to the specific panel above, but talking about the same thing.
Allie writes at length about the importance of visual storytelling to comics. He uses lots of examples, both positive and negative, but one of my favorites is when he points at some Alan Moore comics to prove that it doesn't matter how talented the writer is. If the artist doesn't convey the right information, the comic's going to suck.
This isn't to say that the art is more important than the writing. That's not true, and neither is the reverse. What's true is that art and words BOTH have to do their parts to make a good comic.