Repeating the same things over and over again. If you ever read detective Conan, you know what I mean.
Panels and dialogues bubbles goes everywhere. (I don't want to give examples but you know what I mean)
Plot not going anywhere or no plot at all,
unreasonable character settings (let me just gave you an example, a guy suddenly said that he loves the over person and pretty much stalking and barging into the other's house numerous time etc and then even though the latter try to resist, they're doing it half assedly. Like, what the hell, dude. Call a police! If there's a stung reason for it I might still consider continuing but if it's none and just the author wanting them to get together, nay, goodbye).
And this one is just my own unreasonable condition : if the pairing is not the pairing I've wanted or the uke and seme (on a BL) is totally wrong. And not focusing on the main couple but keep on branching yo the second cp. Like, I read this to know the story of the main cp, not a god*dmn second or third cp. If I want to read it I'll just wait for another series focusing on them.
I think I'm just ranting there. XD
There's some more things that'll turn me off but I only able to think about these for now.