It's a remake plus almost a doubling of items, passive abilities, trinkets etc. As well, more of the passives and trinkets have synergies with one another (for good or ill, depending on the combo). the original game was built in Flash, and, love it as I do, it's glitchy as fuck. it just does things that Flash was never designed to do. When they made the "Wrath of the Lamb" expansion, and tried to compile it for the first time, it just imploded, and they had to spend a few months recovering the code and jerry-rigging it to compile. So, to avoid that, they're basically rebuilt the game from the ground up in a new programming language (no idea which), given it a facelift, and added in a shit-ton of new stuff. AND, if you own the original Issac when it comes out, they're giving people a "ownership" discount.