Hellfire includes a couple Fut Coins dozen new enemies, over 30 new magic items, seven new spells, and a handful of relatively simple new quests. The Monk character class is a very powerful fighter-mage. In addition to being a formidable spell-caster, the Monk is an adept martial artist who can use a staff to attack multiple opponents at once and is skilled in unarmed combat (yep, now you can kick Diablo in the head). The new spells include a healthy mix of defensive enchantments, devastating new offensive weapons (the original game's most powerful spells, Apocalypse and Nova, are now also available as spell books), and spells that just make wandering through the labyrinths more convenient.
Unlike the original game, most new monsters are represented by original artwork and are not just differently colored and powered variations of monsters previously used in the game. But there's a price to be paid for all the new art, as the monster animations and death sequences, in particular, appear to be less detailed than those included in the original game (although there are some great additions, such as the Liches, Crypt Demons, and Orbs).
The new Demon Crypt tileset works well and is as detailed as any of the four tilesets in Diablo, but the Festering Nest tileset is a bit too disorienting and cluttered. There are, however, some nice graphical touches. For example, the barrels that appeared throughout the original game are replaced in the Festering Nest levels with organic body-snatcher-ish pods and by suitably gothic urns in the Demon Crypt. Hellfire adds some impressive new spell effects, such as huge glowing fireballs. The new levels also come with their own musical scores, which mainly consist of ambient sounds and don't measure up to the quality of the music in the original game.