The game no longer chugs along like it’s in pain, except for occasional frame rate drops on the PS Vita. They’ve added new items, levels, characters, bosses, secrets, challenges, and more, overhauled the graphics and gameplay, and added tons of new options. McMillen and developer Nicalis have ported Isaac to a new engine, freeing it from the shackles of Flash and nearly doubling it in size. Creator Edmund McMillen (one half of Super Meat Boy developer Team Meat) regretted his original choice of engine - it seems he never anticipated Isaac taking off like it did - and Rebirth is his recompense. Rebirth is essentially what The Binding of Isaac might have been in the first place, if it hadn’t been developed originally using the outdated Flash engine.
But as you delve further into Rebirth you begin to realize that almost every facet of the game has been improved. In many ways the two versions of the game are very similar - if you haven’t played in a while you won’t notice every difference right away. This was true of the original, and it’s true of Rebirth. It’s important to not lose sight of just how gross The Binding of Isaac is.