Instead, co-creator Edmund McMillen has finally revealed the mysterious new mode he's been teasing. Named Greed Mode, it's a mob/boss rush-ish survival challenge mode which relies heavily on shops to give lots of options in how you build your baby.
Each floor in Greed Mode will contain one of Afterbirth's new double-size shops, two treasure rooms, a curse room, a devil room, and a large room with an ominous button in the center. Press the button and monsters will start spawning, while a timer counts down. When it hits 0, in come more monsters. Each waves brings the reward of pennies. You can stop the spawning whenever you want by pressing the button again, but it'll hurt you and reset your earnings bonus.
Greed Mode relies heavily on shops, see, having them sell a wide selection of items - including boss items - and pickups. You'll need them for health and keys to open each floor's second treasure room, as well as simply for upgrades.
There's a lot he doesn't explain about the mode, mind, like how waves and bosses and devil rooms and item pools and whatnot work. Well, it wouldn't be Isaac if you knew what you were getting into.
Regarding the release announcement delay, McMillen says "I know it feels like I'm really fucking with all of you at this point but trust me when I say that this week's delay was simply due to the inability to nail down the specific release date on Steam and since we are still actively working on getting that in stone."