Definitely art, specifically having a unique and complete style. Meaning, the art work doesn't change over time (Looking at you Hoshino sensei!) even if it improves. This, for me, helps with immersion, especially when you're browsing through hundreds of comics. The color schemes, the style, should all feel like they're part of a world. My own comic would utterly bomb this test though haha. My art style is constantly changing, although its for the better. But that's fine, my comic was, in a large part, a personal project to force myself to improve aspects of my art I have difficulty with - and its working well so far!
Facial expression, is another tidbit I'm not sure if I saw here. Even if its derivative of another artist, there are some comics where you really feel the facial expressions on the characters as opposed to "That shape looks kind of smiley, so she's happy? Maybe?". You shouldn't have to read text to know how a character feels.
Finally, I'm a sucker for interesting paneling. I love Ishida Sui's paneling in Tokyo Ghoul, I feel he really pushes the envelope for visual narrative. I've also seen a lot of Tapastic comics that have creative paneling. That said, it doesn't have to be epic, but it can definitely break a comic if your panels and scenes are not coherent.