There's one new continent, with a dungeon that equals about half the size of the game world in the original game. And new enemies/surprises around the original game world.
Also, your presence on the new island has apparently angered Death itself, so throughout the game after you initially go to Bitterblack Isle, you'll be stalked by him similar to how Nemesis stalked you in Resident Evil 3. He's a perpetual boss, so any damage you inflict on him between fleeing from him will carry over to your next encounter. He can one-shot you if you're not careful.