They made several improvements, ranging from improved performance to better (earned) fast travel options. All the DLC from the original game is included, and the new dungeon adds a lot of gameplay. Considering the expansion plus the core game is only $40, it's seriously a fantastic bargain.
Can't guarantee enjoyment, as some people don't like the combat which is just mindblowing to me, but people like what they like. The game takes the Dark/Demon Souls approach of minimal direct explanation, so don't expect any cinematic flair to the delivery. You'll only get as much story from the game as you're willing to search for. The combat is what really sets this game apart from its peers.