Actually, TheHell has a lot less bugs than even D1.09 (let alone Hellfire 1.01), many of the original game's stupidities and disbalance issues were fixed: black death HP steal, shrine mana rape, nitemare/hell mode too easy start, undroppable uniques, monster AI (which now also varies according to game difficulty mode), chainlite/fireball disbalance issue, monsters now have additional resistance factors like stun resist/immunity, stonecurse immunity (which makes Mages no longer overpowered like they are in original game), life and mana regeneration are implemented (actually it wasn't D3 but us who first got rid of annoying potion consumption routine), players are way better balanced for PvP, so on, so on,...
Many say it's absolutely unequaled, godly Diablo 1 improvement
http://www.thehell.narod.ru