The Hell is concentrated on getting rid of all the initial bugs of Diablo 1, it raises challenge, gives much more variety (monsters, items, spells, boss monsters, unique items, item & environment colors), improves overall difficulty balance of the game, improves monster AI, introduces new character classes, increases replay value and gives full support for Multiplayer experience. It is fully functional, has many bugs of original game fixed, is running smooth, stable, and is much better than original game. The amount of changes is tremendous, part of them are available in Changelog file inside the game archive.
Out of all the mods that I have played, this one is arguably the most conflicting in terms of fun.
Pros:
The new items, new monsters, new ai, and new characters make this game a hoot. The gladiator and assassin are incredibly fun classes to play with the duel wielding theme. The monsters make the game almost feel like something totally different. Bone demons, flesh things, and immortals add a ton of depth to the new beastitary. Also the plethora of new items and weapons just made me giggle with happiness.
However, the difficulty of this game makes it boring after horror difficulty. Yeah I know, the mod makers wanted this to be hard and challenging. But when you have to play the same level over and over again until you get some adequate gear for the next level, then it simply becomes dull and unexciting. Plus the fact that you die after three hits (even with good gear) from the weakest of monsters on purgatory and after makes this game really unattractive.
The last offender is also the requirement for some of the better gear later in the game, in which acquiring new levels to add stats for that item is a long and painful process.
Over all this game is good, but not excellent like everyone is lauding about.