The Binding of Isaac was never supposed to be a hit. It's a game Edmund McMillen threw together at great speed and low budget, collaborating with Florian Himsl for the Flash coding, and the theory was that it might sell to a few thousand people, maybe. So of course it went on to sell a couple million copies, earn a free expansion pack, and inspire a dedicated community. The universe is a sarcastic place.
It's also a place where The Binding of Isaac could never reach its full potential because the game's programming language, Flash's ActionScript 2, simply isn't up to the job. It's great for a quick little 3-month project you just want to toss out there as a brain-cleanser after the development hell of Super Meat Boy, but not so hot for a multi-platinum selling game. So, in the original game's post-mortem, McMillan announced The Binding of Isaac Rebirth, a complete remake and expansion that would see Isaac reach its full potential. It's running a bit later than the planned release of early 2014, but in a Q&A on his blog today McMillen explained that not only was Rebirth coming along nicely but also filled in a load of other details on what the update entails.
The first and most obvious change, which was announced long ago, was the update to the graphic style. The original art was kicked out quick, like everything else in the game, but McMillen simply isn't a pixel artist so the "flash style" had to do. Personally I like the hand-drawn look, but once more work has been done on the GBA-style visual update it will have its own charm. Current screens aren't showing fog, lighting, smoke, and other bits of moving scenery. The old art is being left behind in the old game, which will still be available for those who really can't live without it.
Other than looking nice, the new engine is also not-Flash, so it supports proper control configurations like just about any other game released in the last decade. Also proper screen resolution options, almost zero slowdown, local co-op multiplayer, shared seeds so you can compete against a friend's dungeon, the ability to show off Isaac's final form, and other lovely-sounding goodies that simply wouldn't be possible in the original engine.
https://www.anrdoezrs.net/links/3607085/type/dlg/sid/UUhgUeUpU55141/https://www.youtube.com/watch?v=Z6_C2ppkdVc
The biggest upgrade, of course, is in all the new content being stuffed into Rebirth. It's almost, but not quite, twice as large as Binding of Isaac with the Wrath of the Lamb DLC included. Current plans call for three new characters, 150 new items, new tarot cards, interactive items like the slot machines, and of course tons of secrets. In fact, the secrets are half the reason Rebirth isn't landing on Steam Early Access, because letting some people start mining for hidden goodies before most would want to buy it would spoil the game. The other half is, of course, hating to show off a work in progress.
At the moment there's no set release date for Rebirth, aside from an undefined point in 2014. It's currently planned for release on PS4 and Vita, free Plus users, and on Steam. Owners of the original Binding of Isaac will see a nice discount of some sort, although specifics are still under wraps. There are no announcements about other consoles at the moment, although McMillen would like to bring it to 3DS if Nintendo ever relaxes their restrictions on religious content.
There were a few other bits and pieces in there as well, but my favorite answer had to do with DLC, specifically of the Day-1 variety. The Q&A has all the details, but I'll let Edmund McMillan drop a universal gaming truth to wrap this up-
Q: Will you be adding day one DLC!?
A: NO! You have no idea how many times this is asked, the game isn't out yet. Only assholes plan day one dlc that isnt included in the games release!
Damn straight!