Well, you see, when I first ever played Diablo, it was already with Hellfire, and it was called Hellfire. And then I bought this Diablo 1 a while ago and I finished it and I was like "WTF is that bomb? Where is the tomb level with those banshees... this game is crippled!". I just think original Diablo is too short and small. And it kinda lacks some items. And you can't switch difficulty levels in SP Diablo as it turned out. It's not like Hellfire is absolutely necessary but without Hellfire the game just finishes in a few hours...
To AlphaX
D2 is pretty redundant SP. Diablo1/Hellfire did a better job.