These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Base Breaker: Izumi's treatment of Mei has made her fairly divisive with fans - mostly regarding whether it was justified or not.
Ass Pull: The reveal that Mikami is the Dead Person and the fact that she is, actually, Kouichi's aunt.
Not to mention that ALL the characters already knew that the two of them were the same person, and that she attended that same class in the past and yet NONE of them bring it up, ever. One would think that would be important considering the situation they're in...
However, it's extremely rare to see a substitute homeroom teacher. No other series have bring up anyone. Her death scene in the anime gave us all the foreshadowings.
It should be noted that this whole detail is handled much more gracefully in the original novel.
Broken Base: Regarding the ending, and generally split into three groups; those who think it was just as good, if not scarier, than the rest of the series, those who think the quality isn't as good but is enjoyable nevertheless, and those who think it was terrible and relied too much on gore rather than mystery.
Big Lipped Alligator Moment: The random dream sequence with Kouichi in the middle of Episode 6. The apparently kindly innkeeper Keiko going Ax-Crazy for no real reason beyond an off-hand mention of grief over her grandson's death a good month ago.
Cosmic Horror Story: The curse is outright stated early on to be less of a curse, and more of a force of nature. The best guess is that it's trying to even the books by eliminating the person in class that shouldn't be there, but is indiscriminate in choosing its victims. Add that to the fact that it is never explained what it actually is, and how they only stop it for the rest of the year. It will always return the next year...
Harsher in Hindsight: In Episode 3, we have Kouichi, Teshigawara, Kazami, Izumi, Yukari, and Takako all discussing their futures. With the very nature of this show, only Kouichi and Teshigawara are still alive at the end. Also we start to see the hinting of the romantic feelings that drive Kazami and Izumi mad.
Izumi teasing Nakao about being carsick becomes a lot less funny when his anuerysm bursts and he dies.
Hilarious in Hindsight: Minor in-universe one. In Episode 8, Class 3-3 splits in to teams to catch food to eat. An octopus sticks to Mei causing her to panic. She claimed it was trying to eat her. They end up eating it on the BBQ.
Ho Yay: Mochizuki and Teshigawara are almost never seen without each other.
Memetic Mutation: Pies. Explanation (Spoilers) After someone photoshopped Nurse Mizuno smashing her head into a pie instead of the elevator's floor, photoshopping images and creating GIFs to include pies in some way has become very popular. "Slice of pie" has even become a euphemism for death on 4Chan.
For some, Sakuragi's death goes from horrifying to Black Comedy when you see her hand in the air and her foot leaning on the last stairstep, both twitching in an almost comical manner. That was a bit too much.
In episode 10, it's revealed that the calamity was stopped by killing the extra student. Teshigawara says, great, now we just need to figure out who the extra is! Then—he's interrupted by Kouichi and a needlessly loud background track.
The... almost the entirety of Episode 11. It's packed to the brim with Shout-Out to other horror movies of the splatterhouse variety. The deaths are so ridiculous and the logic nonexistent that the audience can't help but laugh.
Then again, the whole point of the curse is that when death itself is coming for you, not even the laws of physics, probability or logic will save you.
In Episode 12, the chandelier falls on a group of students trying to escape. One of them is unharmed and gets out. He runs for the door... and is immediately crushed by a random falling pillar.
If they cook with gas, plenty of explosions are to be expected in the case of a fire.
Holy god, the end of Kouichi's "dead one" Catapult Nightmare. The melting seems more like something from a comedy anime.
Paranoia Fuel: Some of the deaths seem designed to show that no matter how careful you are, there is no possible way to anticipate and defend against every one of the infinite ways you may die every single day.
The Scrappy: When compared to his extremely minor role, background character Kenzou Kawahori has a large amount of hatred. He does two things in the entire series: First, he calls Kouichi an asshole for not wanting to participate in a witch hunt against Mei. Second, as the Sakitani Kinenkan is burning down, Yukito, Sayuri, and San get trapped under a chandelier. He runs away without helping them. Nobody complained at all when a pillar crushed him, and the other three survived.