1) Too long. I did like One piece, honest but but 200 chapters later with no end in sight...
2) Too much fanservice/Ecchi
3) Characters that refer to themselves in third person
4) Female harems. I don't mind reverse harems, but that is because they usually have something else going on, some long term conflict that is the focus,where the harem is a subplot.

1) If I ever drop a manga, it's usually because I have huge issues with the main characters. If they're boring, stupid, or cliched main characters I get super annoyed and no matter how good the plot is, it's not worth watching these idiots screw up. The ones that try to be heroes just for the sake of saving people because that gets old really fast or the ones that are completely perfect because if we know they're going to win in everything, it just kills all tension. The ones that are so naive and gullible being messed with time and again pisses me off so much whenever I see them. No one's really like that and it's just so boring to read about these characters.
2) I also really hate fanservice. It's super distracting and just detracts from the overall story. I avoid manga that's in the ecchi genre because of this. I'm a girl, so it's really annoying to have to read a manga and see girls with enormous boobs and panty shots.
3) Harems of any kind. Why can't people just stick to one lover!?
4) shotacon and lolicon. It's really disgusting to see a kid who hasn't even hit puberty being molested by a guy that's at least 20-years older than them.
5) Filler chapters. They make it pretty obvious that the mangaka doesn't know how to continue the plot.
6) If it's really really REALLY long. I stopped reading Bleach because it took at least 40 chapters to get through one arc and I'm not even going to start One Piece because it's at least 300 chapters long. Then when the manga finally ends, the ending makes absolutely no sense or is doesn't tie anything up.
7) When the mangaka just starts killing off characters for no reason besides spicing up the story. That just makes their deaths pointless and doesn't move the plot forward at all. I hate it when one of my favorite characters just dies and everyone mourns him for like 10 seconds and then they completely forget that the character ever existed.
8) I know I said this in the first one, but I can't stand the cliche shoujo main characters. Shoujo main characters are usually gullible, naive, pathetically good-natured, and really really weak. I end up reading shounen which have main characters that are no better, but at least they're strong enough to defend themselves. Why are a lot of female characters really weak? I always practically whoop with joy when I come across a manga with a strong female lead.
9) Actually, I don't even read shounen that often because the main characters are also stupid and cliched and there's so many pointless battles, I get sick of it really quickly.

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Agree with the person above me, well, except of of the shota one lol.
To add:
1. I hate angst, too much of it is just plain tiring
2. I hate it when a pathetic insecure frigging shoujo typical girl gets, well, to insecure, like "omg, he's talking to her, he must like her better than me", etc, etc
3. I'm not a big fan of unrequited love unless, the character finds another better love interest, instead of going back to their unrequited love.
4. I really hate it when a girl/guy ends up with a douchebag character, or even leaning that way

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Off the top of my head, here are some of my pet peeves

*Annoying side characters whose only role is to interfere with a romance between the two main characters.
*Excessive nosebleed humor whenever a male protagonist sees a nude female
*Bad endings in general
*Too many characters
*Pathetic protagonists
*Shotacon/Lolicon

bad construction - where the dialogue all over the place, the panelling and art too cramp, and the width of the ink is not differentiated so everything look flat. i can take the cliche-est of the cliche-est manga, but just please make the reading experience an easy one for me. i don't even ask for good art, just a serviceable one is enough.

It depends what I'm looking for....

But in general

*ecchi
*gag boobs
*harem with fickle or pathetic lead
*Mary Sue/Gary Sue

(Depends actually.
But I mean, come on, I understand being multi-talented, but everything? Being simply proficient or average in multiple areas is not only more realistic it leaves room for much more plot.)

*Wangst
*friends that are easily forgiven, to the point where it's like this: "You blew up my grandma, threw away my clothes, dumped me in an alligator pit, punched my cat, and farted on my face...but I'm going to give you one...maybe eight more chances.* mega drop

I tend not to read anything that has those, but I might give ecchi a chance especially if it's hardly present.

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I like my men androgynous.
I like my women androgynous.

Well, mindless/plotless ecchi, etc...axed endings/rushed endings...like, if I feel it could have been done better. I think.

- Anything that screams shōjo is a total turn off (stars and bubbles apearing out of nowere, hair that defies the laws of gravity etc.) (see: Sailor moon)
- Shota/loli-con
- Female characters (esp. main) that are meant to be moe, but are really just ditsy and absolutely useless (i.e.only there for fanservice [because breasts})
- Series that go on FOREVER (Bleach, Naruto etc.) (I mean, c'mon, you don't need 20 books to complete such formulaic plots...)
- Formulaic/overdone plots or plots that take ridiculous turns that don't actually make sense.

I'm sure there are more, but those are my major turn-offs.

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- weak, pathetic MC
- stupid misunderstanding that break relationship
- female character that supposed to be strong but then be useless, etc
- shallow friendship, easily broken by slight misunderstanding/jealousy, etc
- any Deus ex Machina plot

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-shoujo guy who's SO cool/perfect at anything!
so far, that's what made me drop the series since i can read almost evrything..

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Weak main character
Bad art
Bland story line
Annoying male lead/ Female Lead

Unexplanatory inconsistency. Disgusting art. Horrible plot devices.

recently i lost interests in reading mangas...i dont really know why though. i have lot of mangas on my reading/wish lists but i dont read any of them. but things that put me off are when the story is nonexistent or its just stupid and when the manga drags on and on (like bleach and naruto and fairy tale). but basically i just lost interest on reading mangas, all seem the same to me, it could be because of the mangas i like to read have been done lot of times and it bores me now. now i just watch Korean dramas and read books (recently bought curse of chalion trilogy).

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-stupid misunderstanding that break relationship, is what i hate the most

made me remember about one manga(don't the name), that the female lead, that when enter high school she like one guy, but then end up with his best friend(cuz he was the one that help her out with everything, while the other one kinda ignore her, good choice tbh), but then because of a misunderstanding she goes back to the other guy(that she liked first) and end up getting pregnant of him, that guy leaves her(saying that is not his baby, he was a bit confuse he later realize is mistake, but still piss me off), after that she loose the baby in the begging of the pregnancy(don't remember the reason), and she get to know the true about the misunderstanding, i don't remember how it end it =(

-Main char that is weak and don't think for themselves
-Too much ecchi
-Stories with rape/molest

that is the worse, just make me want to turn into a 2d char to kill all that bastard that rape/molest someone, even so i can read them but if they dont focus on that, still i dont like, exemple GTO i love the anime is AWESOME, but the manga have some dark stories still i love the storie minus that small parts(the example of GTO is the Vice-president molesting girls in train, grr kill that bald)

- love triangle between 2 boys and 1 girl,
sometimes 2 girls and 1 boy, but mostly the other way around, cuz most of the time the girl that is weak and end up causing misunderstandings, im not saying that 2 girls and a boy doesn't create any misunderstandings, but i see it less

-reverse harem,
that is probably because im a guy, i put myself in the shoes of one of the char, and "no girl of mine goes around flirting with other guys xD" im kinda of jealous type(my GF to prove it, she is mine and mine alone, grrr xP)

-Harem if is only for ecchi

What turns me off about a manga....

love triangles, there are sooo many, but sometimes if executed perfectly, i like it
too many words, a manga that has so many words that it just covers the whole page, this isnt a novel!!!
girl who falls for guy who treats her like shit, so annoying
typical storyline, like the one above