SNORTS 
Oof, I get where you're coming from but as someone who enjoys geeking out and giving advice and feedback when it's asked for, I kinda enjoy those threads sometimes...
I personally don't have a huge problem with any kind of thread because I guess I just... Don't interact if a thread is not for me. Sometimes it's a bit annoying to scroll past a lot of promo threads with no theme to find something interesting to discuss, but I don't think the existence of this kind of thread is the problem here, it's the amount of pretty much identical threads (again, I'm talking about the uncreative ones with no theme). Same goes for sub 4 sub. I have my own opinions on sub 4 sub, it's not for me, but I'm not here to police what other people do. But again, the amount can be a bit overwhelming.
It's nothing more than a mild annoyance though. Like, scrolling won't kill me and it's very easy to just not click on those threads.
Just thought of another: the same person uploading a promotion post announcing every single update to their comic or novel, thus creating more "link dump" threads filled with the same group of people (those looking to reach milestones, sub4sub, and general self-promo)
At this point I should just mute all promotions since I don't even bother to click on most of them.
Posts that are unspecific shouts to get attention. It usually goes along these lines:
- I can't believe this happened!
- So...this is weird.
- I have an important question!
But, it's the internet, this type of stuff is expected. Also, if too many promo posts are showing up, I just search a specific category to avoid promo posts.
See what you mean but gonna say this; Not all people are secretively negative to their core and the niceness seen isn't always a sort of act to be "socially acceptable". That's just who they are; while yeah, some people do it as a act for sure, that's not always the case for all people in general.
Can somewhat understand how it can be seen as off putting for people begging for favors to strangers without anything in return, but on my end its never a super negative "What in the bloody damn hell is this lazy f doing wtf..." ; its more so I see the thread and bypass it to find something a little more engaging for me.
I mostly look into Collaborations category.
There's so much bad unpaid collaborations thread, asking way too much of artists, and not offering a single penny (rev share is out of question). They ask artists to do 90% of the work, long term, for a story that the artist have no say in. The writer could at least take some load off the artists by helping in storyboards. Storyboarding doesn't need good art. Heck they could storyboard with a mouse and ms paint.
Well, the fact that I've got the whole "Promotions" category muted speaks for itself, I guess...
Though I also check the forums from my phone (which won't let me log in no matter what I do), so I still get to see promotion threads from there... and man, the amount of spam/sub4sub is unbearable ._.' as many others mentioned, the problem with those threads is that pretty much everyone just drops a link and never ever bothers to check what the others posted, so it's literally useless. I've gained more readers from simply posting my art here on the forums than I did from dropping my link, tbh.
Also:
-"Hey so I recently got an interest in writing and even though I never even wrote anything before there's this super vague story idea I have which I think could become the next big thing. I don't have an outline, characters are still to be decided, the plot consists roughly of three lines. I'm gonna need an artist who draws in the same exact style of [insert pro manga artist who's been in the industry for 20+ years]. 400+ episodes, one episode a week. [UNPAID]"
-"WHY ISN'T MY COMIC MAKING ME RICH ALREADY????!!!" (I mean... look, I get it, we'd all love to make money with comics. But if you think that posting a comic online is gonna make you filthy rich after two weeks... I'm afraid I have bad news for you °°)
Well posts that go like:
성인용 및 성인용 콘텐츠를 보려면 video.org에 가입하고 모든 친구를 초대하세요.
헐 이거 번역해주시면 감사하겠습니다
((Disclaimer, this is not an actual link, it's just a text I translated to Korean as an example xD
As a prideful writer, I am disheartened whenever I see misguided so-called 'writers' type up collaboration posts. Yes, producing comics can just be for the fun of it, but there are many artists that really put a lot of their heart and soul into their work and really don't deserve to deal with half the crap that is out there. I understand, many of them may just be young, inexperienced, or not so good at English. But people really need to take a hard look at themselves and their abilities out of self-respect. I'm only saying this, because most of the 'writers' don't even take the proper time to concoct a reasonably structured post and yet have a guts to request help from hard working artists.
But yea, I also don't like those purely sub-for-sub posts either. People need quality subs and a loyal fan base that appreciates their work, not an army of rocks.
As you mentioned, it would be refreshing to see more off-topic posts, or any other kind of discussions not solely related to promoting our stuff.
everytime I see 70/30 once the comic earns, it always seems so bogus to me.
like come on, if they value writing they should value art too, just not for free.
Like most people sub4sub and its varieties. I also get tired of the posts that literally don't have a point. They are either giant essays about a vague topic or are vague about what the OP is looking for in terms of feedback. I don't mind giving feedback or voicing my opinion on a subject but I like to know how to phrase my responses.
Venom starts splashing out of my pores as a natural body response when I see these threads 
Hope we will get a better moderation system to avoid them ^^
i've actually been really enjoying the character questions threads that pop up even though i don't participate in them often and i'd honestly like to see more in a similar vein that motivated folks to think about their characters personalities and what not in depth
threads that dive into worldbuilding would be really cool too. i'm not sure how one would go about making one but idk stuff that gives inspo in terms of ways you could flesh out your stories world
lore threads would be cool too. something like obscure lore from your story or lore bits you've yet to reveal but with emojis or memes or something because those are always pretty fun.
i'm not too big on art trades tho that's more a personal things but casual drawing "challenges" would be nice. something like a color prompt or a word or something just to kind of serve as a collective warm up for those who wanna hop in on it and maybe on the other side something like little writing exercises to practice stuff like dialogue or how to describe stuff and so on
i won't bother getting into thread types i'm not fond of since i think we all tend to come cloaws out at a lotta the same stuff so i'd rather give ideas or suggestions to threads i think could contribute to some more interesting or hopefully useful conversations.
I muted the Promotions category a while back 
. I still find new comics from time to time and the forum is a lot quieter (in a good way). Would recommend!!
I'm with @Legendofgenii - I always get a little bummed out reading big complaint threads about this or that genre/style.
I also enjoy the character threads that ask questions because then it's not just promote-promote-promote. Having a question or a game makes it fun! 
(I really love worldbuilding/lore... maybe I could think of thread topics eventually for that.)
I dislike the threads where it's just people trying to promote specifically their work. Like its not even a share thread but just their work. I'm sorry but no one deserves attention on their creations just for existing. Everyone here works hard and I feel it's unfair when those kind of threads clog up the feed.
It seems like a lot of posts lately are people asking for critique on their series, like they've seen the popularity of some of the review threads and want in on that. But if someone does take the time to read through through their comic and leave critique on it, they either never respond or respond suuuuper defensively. Like, you're the one who asked for the critique! What did you expect? For someone to hug you and kiss your forehead and tell you that you aren't doing a single thing wrong and it's not your fault at all that your comic isn't doing well? It's not because of your bad coloring or your illegible dialogue or your unnecessarily purple prologue, no way!
It feels like "please critique my comic and tell me what I'm doing wrong" threads either want validation and tell them they're flawless victims of an unfair system, or they want one easy trick to fix all their problems and make their comic magically popular enough to make them a ton of money. Instead, they're told that they have a lot of things to fix that will require working and studying and practicing and probably years of artistic growth.
Welcome to the world the rest of us live in! We didn't pop out of the womb born with artistic abilities, we worked at it and studied it for our entire lives, and listened to artists better than us when they critiqued our work so we could improve.