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.
Alternative Character Interpretation: Chitanda suffers from some sort of neurological disorder that affects her memory and prevents her from maturing beyond the mentality of a four year old.
Also, Irisu is emotionally crippled due to being the heir to her family's business, meaning she has to shoulder the burden of running a company before she even graduates from school. The result is that she can't really make any meaningful relationships and comes off as a Manipulative Bitch. She is also secretly jealous of Chitanda's childlike innocence, as she represents the childhood she always wanted but can never have.
Base Breaker: Opinion on the ending of the anime is split between those fans who thought it ended on a logical and satisfying note to those who were disappointed that neither of the USTs were adequately resolved. It doesn't help that the novel series itself hasn't finished yet and the anime only adapted it up to the 4th volume.
Harsher in Hindsight/Unfortunate Implications: Episodes 13 and 14 show students casually wielding a watergun which is a very convincing replica of a Glock pistol. In countries with a history of gun violence, such a thing may be very uncomfortable to watch.
Episode 17 definitely cements Tanabe Jirou as one. He plans and carries out the Jūmonji thefts in an elaborate ploy to get his student council president, Muneyoshi back into manga drawing. To Tanabe's dismay, It fails.
Memetic Sex God: In a certain Image Board, there were some board-goers going like "Hey let's ship him with everyone!" From within the same thread it produced fanarts of crossover couples such as (with the former being the one who leads on) Marika/Oreki and Oreki/Mio. Try looking for them in the archives if you know how to get there, and you'll see. First and foremost warning: said fan arts are NSFW in nature, so use discretion. To put it in simpler words:
"It's been long established that Oreki is instantly shippable with any other character. They're all OTPs before his swag. God help us if he ever decides to expend some energy." — Anonymous
Moe: Eru definitely qualifies, and to a slightly smaller extent, Mayaka does as well.