I first look at the style of the comic--it's not so much about how well a comic is rendered to me, but how well the sense of design is. I often like simple comics, and I expect comics to be simplified, but there needs to be bold graphic design choices to keep me reading. If it's a style I've seen 1 billion times before or if it's just kind of rudimentary, I'll get bored of the art. And honestly, rendering doesn't grip me at all. There's a lot of beautiful comics that are great set pieces but unfortunately aren't well written and seem like...kind of a waste of time honestly. If you want to do concept art, just make concept art, leave comics to comic illustration.
Then I look at how legible the font is--that's pretty big for me. I read on a monitor so I'm a lot more forgiving of small font (and honestly, it looks better, but...it's kind of dated nowadays) If it's a bad font, I just can't continue. I can't read comic sans. I can't read arial. I can't handle most serif fonts in a comic. I'm just so picky about font and it's so hypocritical because I feel like I'm not great at font myself, haha.
Then I look at the words, and I need to feel something about the characters by about page 10 or like 2-3 episodes. If it's that long and I'm just not relating to anyone, or if they just feel like insipid flat character tropes, then I'll probably get bored and drop it. I need that maturity in the writing, youknow?
And then lastly, I need there to be some sort of rising action. There's a lot of comics that don't have bold enough story arcs and story structure, and like--I need actual risk, actual stakes, no matter the genre (except gag I guess, but I don't sub to many of those nowadays). Like you don't need to be a fantasy comic to reference the hero's journey.