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.
Arc Fatigue - The reader is taken out of the main story about halfway through the fourth volume and treated to six volumes (out of a total of 17) worth of uninterrupted backstory - and the scanlations are currently in the middle of that. To be fair, having three volumes about Gilbert's past does a lot to justify how the kid came to be so screwed up, but devoting two volumes to the life of Serge's father after that is perhaps a bit too much.
In story, Serge's fight with Jaques Dren is treated as this, securing his position as the most popular boy in school.
Pascal standing up in the middle of a Latin exam and announcing - in perfect Latin - that it is a dead language and they should be learning more useful subjects such as science instead and then walking out.
Pascal gets another one in volume 12, where, as irony would have it, he gets the highest score in his class and is finally able to move up a grade.
Serge managing to block one of Blough's punches with a book.
Crowning Moment of Funny: That one bunch of students hiding behind their own speech bubbles while spying on Serge and Rosemariné.
Evil Is Sexy: Auguste Beau. Certainly in-story, where he's known as something of a lady-killer; readers' opinions, on the other hand, are considerably more mixed. (He does seem to have avoided Draco in Leather Pants status, however - probably because even his fans acknowledge that he's an irredeemable bastard.)
First Installment Wins - There's a three-novel sequel called Kami no Kohitsuji. Seriously. Good luck trying to find any info on it.
Aside from the Official Couple (and other obvious in-text examples), there's a fair amount of this going on as well.
Rosemariné and Jules have a lot of it. Just to start with, in their very first scene together, the former tells the latter, "You and only you are the one I allow into my heart". And the epilogue more-or-less reveals that Jules had actually been in love with Rosemariné the whole time, but had concealed it under his other (very mixed) feelings about the guy.
Rosemariné also has a bit of it with Serge later on, which doesn't go entirely unnoticed by other students. Near the end of volume 15, when Serge asks why Rosemariné is helping him escape from Lacombrade with Gilbert, he answers, "I like you. ... That's why." The word for "like" he uses is suki, so take that how you will.
Sebastien's admiration for Serge often bleeds into this trope as well.
Gilbert, Gilbert, Gilbert. He may be antagonistic and even violent toward Serge (especially in the beginning), but he sure went through a lot of crap in his life...
While he doesn't hold a candle to Gilbert when it comes to sheer woobiness, Rosemariné nonetheless qualifies once certain aspects of his past come to light. Yes, he's ill-tempered and physically abusive (particularly towards Gilbert himself) but he's also rather pitiable in his own right. Even more so when it's learned that he was physically and sexually abused in his past.
Magnificent Bastard - Arguably, Rosemariné. No matter how much of a jerkass he is in the early volumes, he is just far too fabulous to hate.
Moral Event Horizon - Auguste certainly crossed this line when he raped Gilbert for the first time, so soon after the poor kid had already been raped by Bonnard. Alternatively, he crossed it a little before that with his horrendous victim-blaming when Gilbert had just been brought back home after the Bonnard incident.
Needs More Love - Despite being an award winning manga that helped revolutionize shoujo and was made into an OVA special, the series itself has had little attention outside of Japan and (to a lesser extent) Italy. Even scanlations have only covered less than half of the manga so far.
Nightmare Fuel - Two words: Bonnard incident. Really, though, any scene involving graphic rape belongs here.