For me it's the comic summery, not to be confused with the short 1-liner story pitches. I hate those. This is generally the biggest thing that will make me read a comic, having a summery that sounds interesting or has themes I like.
When it comes to art I'm afraid it's more character design than artist ability. If the characters don't look interesting it's hard to be invested in them, I don't care how well or attractive they are drawn if they're too generic or boring.
Story, generally I'm pretty welcoming to a lot of story concepts and willing to give most a try. If the summery sounds like a fetish or kink though I'll move on to another comic, as I really not looking for those kinds of stories. I do find I love reading a lot more stories with queer characters and poc. You can likely blame the lack of it in mainstream media for that.
After those first impressions and I start reading it very much depends on the readability of the comic. Even if I like the story I can't enjoy if I can't read it, and it just becomes frustrating. Poor font choices, no font choices with everything sloppy written in, and constant use of sound affects that are not sound affects but actions are huge turn offs. Panel and world balloon flow should also be easy to follow and not frustrating. It doesn't have to be perfect or amazing, it just needs to do it's damn job and be easy to read (something I'm discovering a shockingly high number of webcomics can't seem to do...)
The following are now more pet peeve stupid reasons things then legitimate ones that will make me quickly stop reading a comic even if I am enjoying it.
Bad colouring. I know this is a dumb one and it's more of a pet peeve, but I can't take it. It's the absolute worst. I trying to read and my eyes will just constantly focus one the 'corners the fill / paint bucket tool' didn't fill in and the artist never bothered to clean up. It drives me crazy for no reason at all and shows up in even some of tapas premium comics. I f****ing hate it so f****ing much it's stupid how much it pisses me absolutely the f*** off with how f***ing distracting it is.
Poorly integrated 3D backgrounds. Now certainly, these can be done very well and I'm not against the use of 3D models for background, but when no attempt is made to blend the 3D models with the art, or even match the characters to the 3D models (a common one is character will be too tall so it looks like they're be kneeling behind things all the time because they're not matching the size of the object or background they are by or in) then I can't keep reading. It's again, too distracting and extremely frustrating to the point I can't read it.
After that if I read maybe 1 - 3 chapters and it manages to keep my interest, then I will be reading it.