The trailer shows they're adding in a full orchestral arrangement of the OST and 4K support.
I've also heard that they are adding in 'dungeon master' voices to narrate many of the battles and the cutscene portions.
As for new stages and characters, I don't think they're adding anything in.
I'd imagine that the game will perform better on the PS4 as well, but I don't know that for a fact.
I am buying it again. It'll probably be priced reasonably well, and if we don't support releases like this, it reduces the chances of a sequel, which I really want. Their Odinsphere PS4 release was well worth the money and this will be as well. I'll even buy a LE/CE if they release it. Dragon's Crown deserves it.