Goldilocks Zone ... can't be too hard, can't be too soft. D&D used to have a lot of instant-kill magic, which works fine when there are a lot enemies. But in a lot of CRPG's including the old Baldur's Gate, many times your whole party is up against just a single foe, so the developers decided to put in spell immunities which were never contemplated in the classic D&D. That was also when a high-level 8 HD monster had only about 40 hit points. A lot has changed over the years, but you still have to find that optimal challenge spot ... nobody likes cold porridge.
