Specifically, mana for melee classes broke the game against mana-leeching undead mobs (particularly physically-immune such mobs, of which there were a couple in the later acts), because not only did you not have any mana for attacks, you also couldn't leech back life meaning you'd die very quickly.
On hell difficulty there's these gloam things, invisible - or nearly so anyway - while not attacking, very zippy phys immune, unleechable mobs that shoot lightning bolts every 2 seconds or so that hits for LOTS multiple times per bolt from more than a screen radius away. Awful mobs. Run into 5 of those (or rather, NOT run into them since they start shooting before you even come into view of them) and you can die before you know what the eff hit you.
I hope D3 reduces the huge reliance D2 had on leeching items too. It was hopeless trying to survive on potions alone, you NEEDED tons of health leech, and some mana too of course but not nearly as much.