Basically everything, haha. The core demo is the same, but it has new graphics, heavily updated gameplay, heavily rewritten dialogue, Easy Mode + Hard Mode, new features, etc. And some new story stuff added in (so it's a bit longer now, iirc).
So yeah, the core is the same (mostly same bosses and events and such), but basically everything around the base is different.
Overall you'd kinda be getting a heavily upgraded version of the older demo, with a good bit of new stuff also included. It's worth playing if you liked the older demo. :)