The bad news is that you will not be able to engage in any of the new quests until you reach a certain point in the game. Until you reach the ninth level of the original dungeons, or 15th level in character class, you are unable to explore the new dungeon levels. In other words, be prepared to play the old Diablo for some time before even thinking about the new levels. On the brighter side, while you're reliving the Diablo levels, you will have full access to Hellfire's new spells, items, and weapons. While most new spells are variations on the Diablo spells, like wall of lightning and nova, other new ones have almost no connection to older ones, like detect magic and search.
Hellfire has it's share of pluses and minuses.
The new character class is excellent, but the original plan was to implement two. There is a cheat process to enable the unfinished version of the character class, but this is not an official feature of the game. I was disconcerted when I found that I'd have to play at least nine levels of the original Diablo in order to get to the eight new levels. Even when you do finally get to them, you'll find that the new levels are absolutely brutal, some of the new creatures are relentless and extremely powerful. You might even find yourself going back to the Diablo dungeons to gather even more experience before tackling the new creatures.