I couldn't pick just one thing to dislike, so I chose four or five of them.
Use of strong language ~ For me, one of the biggest killers of interest in anything is foul language. I know some authors mistake lots of foul language for humor or a sign this is a mature story, but it fails for both. Swearing as humor only works if the swearing is rare, an exclamation point that quickly pops up and then just as quickly vanishes. Otherwise, it's a failed attempt at shock humor. Swearing to make something sound more mature actually has the opposite effect, making the characters sound like evil roughnecks I'd rather call the police on than take seriously as characters.
Excessive violence ~ Another killer of interest for me. However, this has to be pretty bad to qualify. Looney Tunes violence isn't that extreme because, no matter what happens, you know the characters will bounce back in the next scene either unharmed or with only token injuries. Happy Tree Friends teeters over the line. Sure, it's played for laughs, but the lovingly detailed gore is off-putting. Forget guro.
Strong, radical political, sociological, or religious themes ~ While not impossible to pull off, these aren't easy. All too often, the author just uses them as a platform for shoving his/her viewpoint in the reader's face and degrading anything that disagrees with them, all the while ignoring or minimizing the flaws in their viewpoints. There's a good reason why it's said that discussing politics and religion are good ways to lose all your friends.
Excessive sexual themes ~ I want a story, not a hodgepodge of pornography, thankyouverymuch.
Too much drama ~ Drama is heavy, very heavy. It weighs down the narrative, the characters, and, worst of all, the readers. Too much drama, and the readers will be so emotionally weighed down they'll crawl off to better sites to lose that weight. A skilled author knows to break up the weight of dramatic tension, usually, but not necessarily, with a bit of comedy.
Thoughts on Others ~
Walls of text ~ Sometimes having characters stand around doing a lot of talking is necessary. Using a wall of text is a sign that an artist needs to rethink how they're handling the dialogue. If they can break it up and use interesting camera angles and techniques on the characters talking and standing around, they won't end up with a wall of text. Otherwise, they appear to be rushing the pacing and trying to cram too much material into limited space.
Mixing and matching genres ~ This is doable, but there needs to be some coherence built in before the apparent mixing and matching (perhaps with Chekhov's guns) to ensure that it doesn't come off as abrupt and haphazard. "We're just a bunch o' normal farmfolks goin' about our business...hokey smokes, Batwinkle! Aliens!" works better if there's hints that aliens exist in the story world, even if they're just subtle hints the readers won't pick up on until the aliens appear.