- Anti-Climax Boss: While The Witness, the final boss of the Alternate Route, is undeniably an awesome boss, it lacks one thing that the true final bosses from Afterbirth and Afterbirth+ had: scaling damage resistance. Because of this, it's possible to steamroll through it with the right items; for example, a familiar that can home in on and attack enemies, like a Level 3 or 4 Meat Boy. Worse, if you managed to even get to it in the first place, chances are you're powerful enough to just stamp through, so steamrolls are more likely than a true Climax Boss fight.
- Awesome Bosses: The final boss of the Alternate Route, The Witness, an undead Mama Gurdy. The mod drops all pretenses and flat-out indulges in insane Bullet Hell, with grids of bullets, lasers, shock waves, and enemy summons. And once you've depleted its health, the boss' head bursts out of its body and keeps attacking!
- As far as regular bosses go, the Rotgut is pretty awesome too. It starts out normal enough, with you fighting a Sarlacc-esque monster that has admittedly tricky-to-dodge attacks, but once you deplete its health, it sucks you into it and the fight continues inside of it, in a 2D Crawl-space battle! First, you have to kill a giant maggot inside of it, and then you have to kill its heart; killing both causes it to spit you back out, and ends the fight.
- Awesome Music: The mod has all original music, even for the vanilla floors, and they are all of professional quality.
- Ensemble Dark Horse: While most of the bosses introduced by the mod are well-liked for being creative and unique, some stand out more than others.
- The Rainmaker, the flamenco-dancing, water-bending poltergeist boss, earned quite a few fans for his quirkiness. Being adorable also helps.
- The Siren earned a fair amount of popularity, partly for her for her unique gimmick of stealing your familiars and partly due to being deemed a Cute Monster Girl by players.
- The Scourge is gaining a following for its Ugly Cute design and tricky fight.
- Game-Breaker: The Red Key. Using it can create an entirely new room. This room can be anything from a regular enemy-filled room, to an item room (even on floors where they wouldn't normally spawn), to a Devil Deal, and so on. By creating enough rooms to leave an invisible 13 x 13 grid, the player can even create an I AM ERROR room, and skip to the next floor note . The real kicker comes in when you do this in The Chest or the Dark Room, which resets the floor... allowing the player to get more items and repeat the process, ad infinitum.
- Genius Bonus: Some of the new unlockable items fall into this:
- Akeldama, which is unlocked by Judas after defeating The Witness, is a pile of entrails that lets Isaac leave a trail of bloody tears that hit foes. It's a reference to Judas' suicide in The Bible: he hung himself in Akeldama, and when he died, his organs spilled out of his body.
- Donkey Jawbone, a new item unlocked by defeating The Witness with Samson, is a nod to a story from the Bible in which the mythological Samson used a jawbone exactly like it to kill a thousand Philistines.
- Also in the Bible, there's the story of how Jacob tricked Esau by offering him a delicious stew in exchange for the birthright to their dying father's possessions. In the game, Red Stew gives a short-term benefit (refills health and provides a great, but temporary, damage boost), while Birthright has a different permanent effect depending on the character that picks it up. In the case of Jacob and Esau, the one who picks it up copies three of the other's passive items.
- Hilarious in Hindsight: About a year after Antibirth released its "two characters in one" package of Jacob and Esau, The Binding of Isaac: Afterbirth+s final official update included its own "two characters in one" package in the form of The Forgotten and The Soul. The major difference is that Jacob and Esau requires controlling both characters simultaneously and both differ from the rest of the cast primarily in terms of stats, while The Forgotten and The Soul function more like a Tag Team and have individual quirks that make them much more different from other characters even beyond the "two in one" aspect.
- Nightmare Fuel: Hey, what'd you expect?
- The new chapter, Corpse, is the unholy love child between this and Nausea Fuel. It's the Womb chapter, but after the destruction of Mom's heart. As a result, the entire level is dead and rotting, and is teeming with enemies that are either rotting away (with visible skeletons) or feasting on the decaying meat. And let's not even get started on the music that plays, which feels like something out of a Creepypasta.
- The music that plays after killing Mom's Heart on the Alternate Route, but before entering the Corpse, "Spinning Intensifies". It may have a silly name, but it sounds like Lavender Town, HILL Act 1, and the Reverse Song of Healing all rolled into one. It's almost as if the game is telling you "you've made a horrible, horrible mistake" not completely unfounded, since you just killed your mother.
- The new items naturally do not skimp on the Body Horror, but the absolute crowner is probably The Intruder. It's a little spider familiar that wedges itself into Isaac's face, with its legs poking out of his eye sockets. It also has a chance of making Isaac's head explode upon taking damage, spawning friendly spiders and freeing it to jump at enemies, which is actually less horrifying to look at.
- That One Attack: Baby Plum has an attack where it spews a continuous stream of bullets while propelling itself in the other direction at high speed, bouncing off of walls. This attack leaves you with very little room to dodge, especially considering that Baby Plum is a Basement boss.
- One of the Scourge's attacks has it sucking in bullets from the walls of the room, forming a dense pattern for the player to dodge. The catch is that this attack also sucks in the player themselves, restricting movement and making it harder to avoid getting hit.
- That One Boss: The Heretic is an advanced form of The Haunt, and he lives up to his predecessor's reputation and then some. He starts the fight sending five flunkies with homing bullets at you potentially annoying, but not difficult. But when they go down, The Heretic himself starts attacking, and pain ensues. He can charge at you repeatedly from alternating sides of the room for a full heart of damage, and to make things worse, he will home in on you slightly and can fake you out on the last charge. In addition, he can rain homing explosive tears at you from above, and possesses a Brimstone attack that bounces between the fireplaces in his boss room and will likely take up most of the room. It's not uncommon for players to leave the fight with five or six hearts less than they had when they entered.
- That One Sidequest: The Bloody Mary challenge. You start as Bethany with Book of Belial instead of Virtues, Anemic, Blood Bag for extra starting health and speed, Child's Heart to increase red heart spawns, and there's no treasure rooms. Doesn't sound too bad? Well, you're also given Blood Oath, which reduces your red heart count to just half at the start of every single floor except the first. This would not be a problem for any other character due to soul hearts not being depleted, except Bethany cannot add soul hearts to her health. Or black or immortal hearts. This means that you're effectively a One-Hit Point Wonder starting in Basement II and must pray to the random number generator that you'll get some kind of red heart pickup, and pray again after advancing to the next stage, and so on. Due to a bug, picking up any rotten hearts all-but guarantees death at the start of the next floor, due to the way Blood Oath's health subtraction does not take into account the fact that rotten hearts are always lost in terms of whole hearts, never halves. The goal of this challenge is Satan.
- Unwinnable by Mistake: When you defeat The Witnesses second form of a floating head it turns to stone and falls to the ground, if it happens to be at the center of the room, it will block all access to the chest that ends the run, however it can be saved several ways, either a teleport item which will despawn the boss dead body when you reenter or just quit and reload.
YMMV / The Binding of Isaac: Antibirth