So I can usually tell basically as soon as I click through, there have been some exceptions where I ended up really liking a series after pushing past my first impressions, but like, that's pretty rare.
Thing's that contribute to a good first impression:
1) Art style: This is a lot more than if it just looks good or if I like it though, the style that a series is in can tell you a lot about the creator's influences, and the effort that they've put into the project.
For example, if the art style looks like anime, I can probably expect that the story is going to contain anime influence as well. If it looks like Invader Zim/Johnen Vasquez's work, I can expect that the story is gonna be kinda fucked up and quirky in some way. If it looks like Disney, I can expect a pretty lighthearted adventure story. I'm not saying that if series has X type of style it will have Y type of story, but the chances are that anything that has influenced the creator's art style will have also influenced their writing style
The level of polish that has gone into producing that art also gives me a sense how much the creator cares about their story. High Polish = They probably put a lot of effort into making the story good. No Polish = They probably don't really care about their story. That said, this is not always the case, for example, @Kura and @LordVincent both have art styles with relatively low levels of polish, and they do this because it lets them produce more finished work on a regular basis, which is important cause their stories are REALLLLLLLLYYYYY long. And if they ever wanna finish telling it they gotta work fast.
Another thing to keep in mind is how different the styles of the most recent pages are compared to the earliest pages. Usually, if the artist has DRASTICALLY changed their style (for example, if they were drawing in like, a "marvel"-esque style and then shifted to chibi-style) Then chances are they didn't really have a good sense of what they wanted their story to be, and that it's gonna be a bumpy ride (that said some artists, including myself, use style-shifts intentionally), also I usually won't read a series at all if the earliest art is significantly below my standards, no matter how good the later stuff is, cause chances are the writing at the begging had problems too, and that means that the writing, later on, is built on shaky foundations.
2) The description/premise pitch. Usually, this is less about if I find the idea intriguing and more about how well that idea is initially presented. I believe that just about any idea can be done well if executed appropriately, (and vice versa) and your description is a good way to see how you execute your ideas quickly.
Is your description 6 paragraphs long describing how the world works? I probably won't read it cause 1) it shows me that there are gonna be a lot of info dumps. and 2) you don't trust your series/storytelling to naturally deliver that information to me.
For stuff about writing good pitches, I highly recommend the blog QueryShark
For example, the description of my comic Skeletons in the Closet is very brief and vague, because I want to intice you into wanting to know more, also I do a funny text thing to hint at the subtle horror that's in the comic. I wouldn't use this tactic for say... a Slice of Life Comedy, because you're not really reading a slice of life for the mystery.
3) How many subscribers do they have and how long have they been posting? If they have 1 subscriber and have been posting for 3 years, something is very wrong. If they have 2000 subscribers and they've been posting for a week, they're probably doing something right, or they're already established.
4) Do they have another series? If they have one that's completed that's a good sign, it means I can trust them to deliver a good finished story. If they have 6 abandoned comics I might not want to waste my time on a series with an uncertain future.
I know that a lot of people check out Skeletons in the Closet after reading my Minicomic. So yeah, having finished series is good.
5) How does the artist behave? Are they reasonably professional? chances are they take their work pretty seriously. Are they very casual? chances are their project is just a hobby for fun. Are they constantly starting shit here on the forums? I probably won't want to give them my support just on principle, but also chances are that behavior will bleed through into their writing somehow and get on my nerves.
Once an series has made a good impression on me, I'll probably read like... 10 or so pages, maybe the first chapter, to see if I actually like it, and usually, I do. If it's pretty young I'll just catch up to the most recent update. It depends on how much time I have.
