There isn't much to the original D beyond 'Real-time Rogue'. D2 was much, much more ambitious in scope (and much less focused as a result IMO).
What I meant is that at some point they seem to have tried to expand the game from its original premise (simply: a hero, a dungeon, a town). Then they instead cut what little they had started to expand and ported them into DII.
Wow, nice work. Of course, putting models, animations and backgrounds together is real labor (and at least the monk and Na-Krul probably involved some significant work), even if it's based on a template, but I think it says something that 'Hellfire' is the last title listed in that article on Synergistic, and that Bliz has more or less pretended that it never existed and has never licensed out their IP to an outside studio since that time (and may not even be outsourcing pre-production cinematic stuff again after that leak a few months ago).
Yeah. I was too young to have too much first-hand memories back then, but my interpretation of Hellfire is that it was more Sierra trying to cash on the success Diablo generated, and Blizzard just having to follow the contracts they originally had to sign. And then they made sure they never signed such outsourcing contracts again.
The general consensus is that Blizz had a lot of ideas when making D1, but decided to release the game as we all know, and a lot of the surplus ideas made the core for what we know as D2.
Following monsters were made but didn't make the release and were cut. They are in the game files. Most of them were enabled in Hellfire:
Unraveler - found in Hellfire and Diablo II instead
The Arch Lich Malignus
Devil Kin Brute
Many other monsters are modified versions of work Blizzard had done. For example the end boss, Na-Krul is obviously an edited model of Diablo.
Many spells are slightly modified versions of original spells:
Ring of Fire
Both Hellfire levels use the same map generation tools the original levels did. They just replaced the graphics. The Demon Crypt reuses the tile set for the Church, but with no shrines and libraries. Also minor changes.
Rumor has that Blizzard made some of the tiles for the Nest, but scrapped them as they reused the idea in Diablo II. (Along fe. the two cut quests, in where the player killed Izual and Andariel).
The new levels have some pathfinding issues because the algorithms weren't changed to suit the new levels.
Do I need to think of more content that was reused content from the original Diablo? Mostly cut and disabled, but you can find them in the game files.
I've known for a long time about the cut content from Diablo 1.
Basically all Hellfire content is content cut from Diablo I. AFAIK only that annoying organic lair (the festering nest) was completely their own creation. The other new level is partially recycled material. They also polished some of the unfinished animations etc. that were cut from Diablo I.