AB+ balance day one was AWFUL
- Ridiculous HP and boss armor on Big Horn. On top of that, he could damage you with his pits before his arms even came out
- Sisters Vis were faster, had an obnoxious amount of HP (more than Mom, despite appearing in the same chapter!) and had no tells at all on any of their attacks so they'd just randomly hurt you.
- Void portals would spawn enemies forever, at a faster rate, and had more HP
- Stonies were faster, more common, and never got tired. You couldn't shoot through them with piercing tears.
- Delirium had no animations and would just randomly damage you for no reason what so ever.
- That Blast room in the Womb which was impossible to not take damage
- That caves room with the spiders and mushrooms that was impossible not to take damage in
- That caves boss room that was a soft lock if you lacked bombs to go down the hole
- Appollyon could restart floors as many times as he wanted with Forget Me Now, and could get unlimited items via Pandora's Box. And don't tell anyone, but this bug still exists for the newly added Mystery Gift. Goat Head plus a couple batteries and you're in business.
- Unlimited lives with Smelter + Missing Poster.
- Greedier mode was basically unplayable. You couldn't even buy an item on the first floor, since you only got like 7 coins total. And remember Sisters Vis basically being a run ending boss? Well imagine fighting 4 of them at the same time.
- Some achievements literally did not even work.
- Pop ups for AB+ unlocks were the completely wrong color for some reason
And even now, after most things have been balanced, remnants of Plus' poor balance still live on. Blisters (the spider enemies that shoot Little Horn balls) still have way too much HP and are too common. They also replace Greed Heads, which is dumb when Greed or Ultra Greed summon random spiders that have no relation to them. Ultra Greedier is still stupid. The Sisters Vis are too easy. Delirium is still pretty janky. Portals are still too common (according to my bestiary, they're as common as Red Flies). Booger tears can soft lock against those worm enemies in the Womb. All of the achievements still share the same boring background and aren't properly color coded like every other achievement in the game. Most of the content isn't behind unlocks even when there's no real reason it shouldn't be (like Appollyon)
But it's a lot better now. AB+ for the first few weeks was a mess. Had they not fixed it as quickly and as good as they did, Isaac might have likely lost a lot of popularity, because it made the game just stupid to play. It wasn't even fun in a challenging way.