Awesome Music: Nate Fenwick Smith did a crazy good job at the soundtrack. (Can be listened to on YouTube, or on his Bandcamp) Some special mentions:
The wonderfully atmospheric "Welcome to Edgewood", which sets the game's recurring riffs and plunges you right into the dark atmosphere of the game.
The theme of the lategame levels, "Time With Your Caring Friends", starts out with a dreary piano tune that soon transitions into a heroic, resolve-filled composition, briefly returning back to a more melancholic tone by the end of the track.
And then there are the boss themes. Two particularly notable ones happen to be "Juice Boxes Required", an intense track that really emphasizes the sheer danger of the bosses you encounter by that point (It doesn't help that at least a good half of the Dorm bosses can qualify as That One Boss), and "Our Darker Purpose", a wonderfully triumphant boss theme that plays when fighting the Library floors' bosses, including The Administrator himself.
Being the only one to survive the massacre of all the quiet kids in the class by your best friends, no less, then having to battle almost every single child in the building — with no trustworthy authority figures around — just to find out what the hell happened.
How would you act if one of your classmates or friends could create living monsters with a blackboard and chalk? According to the kids at the orphanage, acting like nothing and getting killed by them is standard, even before the adults disappeared.
The entirety of Edgewood was a horrible place even before the massacre — all kinds of hostile supernatural phenomena happening that often result in loss of children's lives at least daily, the constantly-shifting architecture of the orphanage itself means that it's very much possible for someone to disappear without a trace by virtue of the room they were in simply vanishing, and quite a few of the staff members were very much complicit with all that was happening, not a care in the world about the lives of children in their care. And you thought regular orphanages were bad enough...
The actual story of the game? It all turns out to be a metaphor for the increasingly deteriorating mental state of The Bandit, the antagonist of an in-universe book series, and it's likely that all of Edgewood is just a figment of The Bandit's imagination seemingly based on his terrible childhood experiences. The True Final Boss is called His Natural Defenses for a reason — after all, it represented the last shreds of sanity they had. The Gainax Ending? It's basically The Bandit finally snapping and succumbing to his madness. Cordy herself? The living representation of said madness, going completely out of control in the aforementioned ending. Of course, one does need to pay close attention to all the Archive entries to find out all of that in the first place...
Cordy's movement speed and her dodge roll are one of the main points of contention in regards to gameplay, as she moves slower than most enemies, often requiring to you use it to dodge enemy fire (or the enemies themselves). However, there are no invincibility frames, either. Popular loadouts often include one of the two roll-replacing perks — either Vanishing or Being Scared.
To a lesser extent, HardMode. First off, it takes up three perk points, making it a lot more difficult to create a viable build and having to sacrifice some perks. On top of that, the game's way of establishing difficulty was not the most creative one - that is, shoving enemies from later levels into earlier ones whenever possible, giving some bosses downright unfair attack patterns (making BreatherBosses a lot more difficult, and making some ThatOneBosses complete run-killers), and is overall loaded with Fake Difficulty. It doesn't help that the new room layouts, which most players are very likely to even bother with Hard Mode for just to see all the Mind Screw described in a certain Archive entry for themselves, are painfully rare, and when they do show up, it's usually at the worst of times.
As if Cyndar's Timekeeper wasn't a difficult enough boss already (see That One Boss entry on them below), the achievement associated with it, "Time Waits For No One", requires you to defeat it without using any of the time bubbles it spawns. Already a difficult enough task as it is, given that without the speed boost provided by said bubbles helps deal with the absolute Damage-Sponge Boss that the Timekeeper is, it's made a lot more difficult by one simple fact - the bubbles' spawn isn't even remotely telegraphed and you may end up running into them by complete accident while trying to dodge the boss' attacks, ruining your chances at the achievement. And as if to add insult to injury, Regret, the perk that is unlocked by the achievement, is only marginally useful.
Generally speaking, a lot of the achievements tied to this or that specific boss are either incredibly difficult (defeat The Friendly Foot Stools, a very erratically-moving boss without taking any damage, defeat Fortress Woundwort without destroying any structures that heavily limit your dodging space and constantly pelt you with projectiles...) or are outright Violation of Common Sense (the Cyndar's Timekeeper achievement above, defeating A Leafy Oversight after letting enough spores return to the point where the boss' attacks become completely unavoidable and the fight degrades into a dps race, defeating Dr. Bloodfather, a boss that constantly drains your health, while you yourself are on single-digit health, etc...)
The Dorm floor bosses are all strong contenders (save for maybe The Virtuoso), but the two that stand out as run-killers are Cyndar's Timekeeper and The Disciplined King. Whereas the former is a Damage-Sponge Boss of the highest caliber that has an incredibly difficult attack pattern and a gimmick that was only really put there to justify giving it absurd amounts of health (not to mention nothing about the boss' attacks lines up with it's respective Archive entry), the latter is a somewhat less beefy, but still very frustrating "Get Back Here!" Boss that doesn't give you much time to actually hit it, and late in the fight starts summoning chess figures that block all incoming attacks and cannot be destroyed.
Regan, the end boss of Chapter 2, has nigh-unavoidable attack patterns (pray that you have some chalk left by the time you reach this boss), is incredibly beefy even by the Terrace floor standards, and moves around rather erratically. Combine all of that with her projectile-absorbing clones that she spawns halfway through the fight and that constantly bombard you with attacks, and you get a recipe for a Difficulty Spike boss that you end up having to prepare for a lot more than even the ones on later floors.
While the Library floor bosses are often considered the better ones of the lot, being challenging but manageable, Dr.Bloodfather is agreed to be an absolute run-killer - spending a lot of time in an invulnerable phase that still allows him to deal contact damage, having nigh-unavoidable attacks, and at halfway health, slowly but surely draining your health, putting the fight on a timer. Not only that, but he is very prone to softlocking, ending up stuck in his invulnerable phase forever if you kill him at the wrong time.
) Some special mentions: