Hmmm, that's an interesting question... I guess when I'm looking to bookmark a comic, I tend to have some criteria, but I'd never really thought about how I'd describe them....
I want the art to be decent, but the thing I rate above it being beautiful or technically impressive is clarity of visual storytelling. There are a lot of webcomics that have beautiful rendering, great colour palettes, amazing anatomical drawing and scenery... but I can't follow what's going on, usually because the facial expressions don't clearly communicate the emotions, or the backgrounds are too busy so you have to search for the characters like a hidden object game, or the panel compositions are just strings of individual characters standing in the middle of frames so they all feel really divorced from other people in the scene, or the angles of shots feel completely arbitrary like the artist has heard you're meant to vary your shots, but has no idea what the function of different angles is in terms of tone and focus, or the bodies are perfectly proportioned, but everyone has the same build and motions and gestures all feel stiff... Basically there's more to being a great comic artist than really pretty characters and shiny rendering, I'm here to read a coherent story told in pictures, not look at a sequence of nice or artsy illos like I'm browsing an insta with speech bubbles. Strong visual storytelling is a big win for me. If it also looks polished, that's great too!
I like a good plot, but characters are more important to me. I'm just not that interested in the epic war that happened 200 years ago between the orcs and the elves, but I might be interested in how it relates to the characters, their beliefs, personalities, and their goals and what's getting in the way of them. I'm not necessarily into slice of life fluff, but I'd rather read a fluff comic with well realised characters than something with a twisty plot full of political machinations about characters who all just read like the same flat, ruthless dude copied and pasted. I want characters with distinctive personalities, speech patterns and appearances I can differentiate at a glance, whose behaviours follow a common thread of logic that makes even their surprising decisions feel like they make sense in retrospect.
If it's GL or has diverse or LGBTQIA content, that's a bonus, but I won't read a bad comic just because it has that kind of content. It still has to be good, and for me to feel that it's a sensitive and informed depiction of my community. Trashy BL comics by straight girls that fetishise conventionally attractive skinny pale gay men and have like... no female characters? Yeah, no thanks.
