I'm evaluating whether or not I should get this rather cheap DLC and I've boiled it down to the two following questions. I'll draw the conclusions myself.
- Does the DLC change the game forever? Is it rather an option at launch or in the settings screen? I know Steam doesn't normally allow you to 'hide' the fact you have DLC purchased for a game.
- How does the difficulty change? Is the game with the DLC objectively harder, easier or just "randomer" than the "vanilla" game is? Is it meant to be a "new-game-plus" content pack for the players who've seen everything from the game?
Best Answer
I will explain everything it adds, so I can back up my conclusion.
There are a lot of things added to the game with it. The ones not really contributing to the difficulty are the Achievements and the Challenges, so I will not cover them.
There will be spoilers in this answer, so if you want to learn about everything yourself, do not read anything below this.
The secrets added are: Samson, D20, Crucifix, Abel, Curved Horn, Sacrificial Knife, Rainbow Baby, Blood Lust, Blood Rights, Bloody Penny, The Polaroid, Isaac's Head, Maggy's Faith, Cain's Eye, Judas' Tongue, Eve's Bird Foot, ???'s Soul, Samson's Lock, the Candle, Lucky Toe, Burnt Penny, Guppy's Tail, A Fish Head, Mega Fetus, Super Fan, Spider Butt, Counterfeit Coin, the Egg Sack, Guppy's Hairball, A Forgotten Horseman, Something Icky, The Left Hand, and Dad's Key.
All of these are linked to either a new item, trinket, or boss.
The new harder alternate floors do make the game more difficult. Overall, the alternates have harder variants of enemies compared to the Pre-WL versions. The hardest implement is probably the Cellar, with the addition of insane spider based enemies, which can ruin any new player's day.
The two new endings are the Cathedral and the Chest. In the Cathedral, you fight a harder final boss, and in the Chest, you fight a harder variant of that boss. If you choose to go down to those endings, the game does get harder (but only if you choose that path).
Six new types of rooms are added.
Angel Room: An alternate to the Devil room, it gives you an item themed after Angels instead of Demons. This item does not cost hearts to take.
Curse Room: These rooms have a spiked door that cost a total of 1 heart to use (half to enter, half to exit)(You can use flying to avoid the entrance damage, but the exit damage always happens). Inside, there are either 1 - 2 red chests, or (if you are lucky) an item from either the Devil Room, the Angel Room, or the Secret Room (Devil items from here does not count as Deal with the Devil).
Sacrifice Room: This room has a spike in the middle. If you take damage on the spike, there is a chance that a chest will spawn. The rate at which they drop though is all randomized (There is also a very small chance for a Devil Room item to drop from this).
Boss Challenge Room: These variants of Challenge rooms always have a Boss Room item in them instead of a chest. But, you need 1 or less red hearts to enter (Soul Hearts do not effect this). Instead of fighting normal enemies, you fight a couple of bosses.
Library: This room looks identical to the shop from the outside, but on the inside, contains two books that can be picked up for free. Like the shop, it requires a key to access.
Super Secret Room: This Secret Room does not follow the normal pattern for Secret Rooms, and is usually harder to find. What is inside depends on the tile set of the room.
The amount of new enemies is staggering, so I won't be able to go over all of them. I will go over some of the important ones though.
Spider and Big Spider: These enemies skitter across the floor in very random movement patterns. The Big variant splits into two the normal variants when it dies.
Mulliboom: This Mulligan with a bomb on his head will bee-line towards you can try to touch you. If he touches you, he will explode, dealing damage.
Walking Boil/Sack/Gut: A walking variant of the previously immobile Boil/Sack/Gut.
Fat Fly: This variation of the Pooter shoots two bullets similarly to Harlequin Baby .
Keeper: A bouncing Greed head, has the same effect of Greed. Shoots similarly to Fat Fly.
Hanger: A flying variant of the Keeper, fires similarly to Greed.
Mask + Heart: The masks are invincible beings that chase you much like Knights. Each one though, is linked to a Heart that spawns with it. It dies when the Heart it is linked to dies.
Sack: Similar to a Boil, but spawns spiders instead of shooting.
There are many new bosses added to the game, many which are "dead" versions of the boss it is based off of.
The list of posthumous bosses are:
The new bosses are:
Gurdy Jr. :Many floors: A mini version of Gurdy, but instead of sitting in the middle, it bounces around the room.
Widow :Basement: A spider that jumps around the room much like Monstro. But, it jumps much farther and much more often. She will spit out spiders, and create White Creep puddles, which slow you down. Often in those creep puddles, she will spawn a Sack. (A relatively hard boss for the floor she is on).
Daddy Long Legs :Necropolis/Utero: This boss hangs from the ceiling. He can cough up spiders like the Widow. To attack, he will either: stomp all four of his feet around the room, raise into the air, then stomp each foot once after another, or slam his head into the ground.
Isaac :Cathedral: This boss sits in the middle of the room, and launches streams of tears to hurt you. When he takes some damage, he will stand up, and shoot homing tears along with his basic attacks. Lastly, when near death, he will start to fly, and "Crack the Sky" (Pillars of light will shoot down, and hurt you), and he will spawn in Angel Babies.
Along with all of those, the 7 Deadly Sins got an upgraded form. They all get a Super variant, which has upgraded abilities.
Super Gluttony: He fires a laser in both directions, and spits out a mass of tears at the player when he does his radial attack.
Super Lust: Gotta go fast! Sprints towards the player leaving a trail of red Creep behind. She will bounce off of walls and move crazily, making it a very dangerous boss to fight.
Super Wrath: Instead of spawning a Troll Bomb and trying to kick it at you, he spawns Super Troll Bombs which home in on you. They do considerable damage to him, so he holds a double edged sword.
Super Greed: He fires 4 shots instead of three, and instead of spawning Hoppers, he spawns Keepers.
Super Envy: Instead of splitting into two pieces each time a piece breaks, he splits into three.
Super Pride: Along with shooting the laser beams, he shoots a tear in each cardinal direction. And, instead of six Troll Bombs, he spawns two Super Troll Bombs.
Super Sloth: Launches out two shots of IPECAC bombs, and can spawn two spiders.
Ultra Pride: Another variant of Pride, this one is not similar to its previous forms. When he spawns in, it also spawns in a (specific) Baby. This baby shoots homing shots (not very often). Pride himself with either shoot out three homing projectiles (that deal 1 full heart of damage normally) or a horizontal line of green Creep.
The four new obstacles appear in rooms and can either help or harm the player.
Cobweb: These slow down the player, and slow down his projectiles.
Hell-fireplace: this alternate fire will sometimes shoot at the player. The projectile is low moving and easy to dodge, but if the player is not paying attention, it can easily ruin a run.
Golden Poop: Similar to Poop, this one drops at least 5 coins when destroyed, or a Trinket.
Corny Poop: This Poop has an Eternal Fly orbiting it, and if the Poop gets destroyed, will turn into a buffed attack fly and try to chase the player.
The curses added to the game do make the game harder.
Curse of Darkness: Removes the ability to see the map.
Curse of the Labyrinth: XL sized floor. 2x Treasure Room, 2x (linked) boss rooms, etc. NOTE: if on first floor, both Treasure Rooms will cost a key to open.
Curse of the Lost: Expands the floor size to a size bigger. If you get it on the Basement, the floor will be sized after the Caves/Catacombs, and so on.
Trinkets: These passive items take up their own spot. They all have an effect (some better than others) and can only get removed if a new one is picked up.
Conclusion:
The change in difficulty though, is more opinion and experience based than anything. A new person, who only played a couple runs on both, would more than likely say that Wrath of the Lamb is a harder version, while a more experienced player would get used to it pretty fast. But, besides that, there is no way to get around the fact that harder enemies and bosses were added, along with a harder ending and harder floors (and even curses to mix everything up). And, seeing that not all of the new items are good, and a couple of the trinkets can ruin a run by themselves (looking at you, Cursed-Skull), Wrath of the Lamb makes the game harder. Having said that, it can be a New Game Plus for those who "completed" the first game, but a new player could start on it and it would not be that different (besides more difficulty). The difficulty spike is not too high, and the learning curve to get used to the changes is very smooth, so anyone could get into it without having to master the first game.
And, for your question if it is forever or not:
Recently, Steam will tell you what DLC you have downloaded, and if you want to de-activate it or not. You will keep anything that was unlocked without the DLC, and if you unlock a Wrath of the Lamb thing, then un-install it, you will keep it unlocked (when you re-install it). You do not loose progress if you switch between versions.
I tested the above out myself.