@Marc NobbsI very rarely stop reading something I had got into far enough, even if I'm actively angry with myself for continuing. A kind of addictive mechanic, perhaps. Rare cases I indeed do, it's because the story "petter out" or perhaps more accurately my interest in it does. More easily and thus likely with ongoing series, I may just never look for the next chapter. Sometimes I may skip forward, if that's an option.
In physical bookstores, it's an annoying habit of me to open the book in the middle first, and start reading, and sometimes reach the end before I'm paused enough to skip to the beginning, then feeling guilty for not buying the already read book...
While most of the thread above focuses on the grammar and representation, that usually doesn't bother me at all. Possibly, for me myself being non-English and thus already having to invest significant mental energy to decipher content; it plausibly might be more annoying in native language, I admit, although can't really confirm that either.
(Haven't read all that much self-published amateur content in my native language; while a "conceptual poetry" book that consisted by 3/4 of increasingly silly and nonsensical disclaimers and then did things like wrapping a line around the outer edge of the physical page... for no apparent reason, was a bundle of hysterical fun. The beef I have with native news media and the like is different, and out of this context anyhow.)
Indeed, unconventional presentation doesn't bother me either, to an extent (I won't normally even pick up, say, second person narrative experiments, leaving that out of context here). But say, a pure stream of consciousness with all dialogue retold in same internal voice with hardly any punctuation? No problem. Indeed, I enjoyed one or two stories conceptually written like that very much. Sure, intentionally doing such is something else than accidentally slipping into almost similar by pure negligence... Just saying, complexity of presentation doesn't bother me as long deciphering the content is possible, I can accommodate and adapt to almost anything. Then, indeed, I can apparently boast about reading comprehension above average, so that could be just me.
I don't have any hard squicks either. In right mood I can enjoy extreme content even despite poor or jarring presentation that often accompanies examples I have encountered. I'm not taken out even ambushed by something totally unexpected if it makes sense, and perhaps even if doesn't.
However, I do have a few "soft squicks" namely, spanking and blowjobs, and humiliation for the sake of it. If the story appears to be going into decaying orbit around a black hole singularity of one of these specifically, I may indeed quit. There's a temptation to generalize it to any one narrow aspect, but I'm afraid it's not the case with some of my kinks (like public nudity) where I would forgive almost anything, even the above (with often, to my great dismay, are indeed mixed together heavily).
There's another, seemingly, but not necessary specific to erotic content, mode of failure to retain my interest by failure to manage (sexual) escalation. I mean, where the story reach the (erotic, to me) apex way before the dramatic, or just way too early, period. All while I know, I'm very much guilty of just that in my own quick fantasies, but for that those are quick and unserious fantasies. While I'm first to insist a dancer keeps dancing after getting naked, and there's in principle nothing wrong in starting a story right there (according to the principle of starting a story as close to the end as possible), the management of escalation is getting increasingly difficult the faster we go, the perceived pressure to go way out past the limits of reasonable is strong, and very often that's a failure point that takes me out of the story very quickly, just when it got going, especially if it's abundantly clear it's going to be a single kink wank from there on and that's kink not mine. To say, I'm not at all so interested in endless descriptions of sex as such.
There's possibly a way to generalize above beyond the erotic. Say, I can easily imagine a "rags to riches" story that gets boring right after first couple thousands earned by the unstoppable money machine protagonist. One may say, the overpowered protagonist is to blame.
Indeed, there was a mage story where there was a well foreshadowed and set up story arc with plenty of interesting and inevitable complications, and then, the protagonist suddenly and accidentally ascended to practical godhood in the middle of an epic figh with equal demigods who come out of nowhere... With no obvious way to salvage the initial storyline it was left unfinished, no surprise, author interest had ended right before mine might have anyway (probably right about, considering what likely was left unpublished and even more unwritten).
However, it's not impossible to salvage and continue even such in interesting ways. In the mage example, it could shift focus on the ongoing merger of magic dimensions and subsequent invasion of the third, all while trying to get along with the unconditional family situation in the mundane world; there even was a plausible way to contain his emergent god-like powers to specific conditions and thus not apply to other aspects or ordinary people, it even could be mighty frustrating for the protagonist and a story arc in own right. Just requiring way much complexity and imagination, perhaps. (I even corresponded with the author; they talked about rewriting last couple of chapters to tone it down instead, but to best of my knowledge, haven't.)
What I'm likely attempting to say, if an author writes themselves in a corner that destroys the story, but then they instead ignore it and keep milling around as if nothing had happened, it's likely to become very boring very fast, and it happens way more often and easier in erotic content and aspects than perhaps others, masquerading easily under sheer veneer of wish fulfillment.
And blatantly peddled wish fulfillment just isn't very interesting without any groundwork and plausible contextual dynamics unless, perhaps, the wishes align perfectly, and even then its not guaranteed to be very satisfying.