While strength is the obvious culprit, the problem extends throughout the system. A character with 18 CON has about twice as many hit points as a character with 14 CON. A 14 Charisma gives you basically a +1 bonus on checks. An 18 charisma gives you basically a +8 bonus on checks. A 14 Dexterity gives you basically nothing, while an 18 Dexterity gives you a +4 bonus and a whole levels worth of improvement to your thief skills. A 14 wisdom gives you basically, nothing, while a 17 wisdom is basically required to unlock the upper levels of cleric just as high intelligence is required to usefully function as a M-U.