Well I'm not trying to sound elitist, but if you thought vanilla Diablo was hard, "The Hell" might not be your cup of tea. The difficulty is upped TONS, however they've added TONS of features.
More unique items then in Diablo 2 there are now 5 classes (Assassin, Scout, Paladin, Gladiator, Mage), shared unlimited stash for all characters, spells are hot keyed to many more keys, more mob models, different attribute types on items, a few new spells. It's almost like a new game with a Diablo skin, that keeps the "dark" feel of Diablo.
To give you perspective I have a level 24 scout and I am still in the caves on the "normal" difficulty setting.