Yes, we are picky.
Sometimes you watch an anime series, and all is well, but then something happens and you just can't watch it with the same vigor. It's like something snapped in you, all because of one reason. We all have our preferences, and these red flags in anime are different for everyone, but users of Reddit decided to make a list of things that are off-putting to them and make them drop the anime series.
One of the most frustrating things in anime is romance, especially the one that features oblivious characters who are in love with each other but refuse to be together for some stupid reason, be it miscommunication or just anything else. The prominent examples are Rent-a-Girlfriend (Kanojo, Okarishimasu) and Hajime no Ippo.
There are, however, good ways of utilizing this trope: it works tremendously when both characters are completely broken and them being a mess is an actual obstacle in their relationship development. Look at Kaguya-sama: Love Is War (Kaguya-sama wa Kokurasetai: Tensai-tachi no Renai Zunousen) — it's a perfect example of this trope being done right. In Tomoe-chan Is a Girl! (Tomo-chan wa Onnanoko!) the opposite works: characters are so believably stupid that you understand that they're stalling not because they want to.
Another romance pet peeve is the usage of love triangles or harems as a way to add conflict to the relationship. One of the victims of that is From Me To You (Kimi Ni Todoke), and the other is How a Realist Hero Rebuilt the Kingdom (Genjitsu Shugi Yuusha no Oukoku Saikenki).
Some state that they hate when the shows with plots more fitting to adult characters utilize teenagers. Case in point: Tokyo Revengers, a show about gangs and delinquents, but the type of the story doesn't fit the 12-14-year-old characters. Gang wars between early teens are different from the ones portrayed in the series; the ones in the show ask for someone older.
Some have troubles with anatomy: shows that draw characters without nipples, be they male or female. One such show is Gintama which features quite a lot of male nudity yet the nipple appears only once (in a severely mutilated state, mind you). Others are put off by an overabundance of fan service and over-sexualization of female characters, like in Fire Force.
A lot of people drop anime the moment they hear the voice actor that annoys them to hell. Quite a lot named Black Clover and Asta, voiced by Gakuto Kajiwara. Others couldn't stand high-pitched voices in Puella Magi Madoka Magica (Mahou Shoujo Madoka Magica).
What can make you lose interest in the series you are watching?