Plenty of my pet peeves got mentioned already, but also, hidden unpaid collaboration. No, "it's paid when we get big on webtoons" isn't paid, it's revenue share, it's probably not going anywhere because you can't be honest with your artist, too.
A lot of threads got mentioned, but there's also individual posts - people who just reply to you with a promo of their comic and run, or just spam post in 10 threads at once with a link and maybe a thumbnail. You're gonna... Tell me why I should like the comic? Why did you feel that you could cash in a really shallow compliment to get me to click the link? Did you even read the OP with the rules (happens a lot with LGBT+ shares and other types of link shares)?
Every now and then one of those users shows up complaining they're not getting any engagement. You pushed away people that might've considered reading before they even clicked a link, of course it's bad.