As I said previously on the odds of each result when rolling 3d6, D&D was a lot more lethal back in 1981. Your character can start with as little as one hit point.
Another odd quirk of the Basic D&D rules, which you can see in Labyrinth Lord's rules (free PDF download here), is that characters with high ability scores gained bonus XP. Since you rolled ability scores before choosing character class, this encouraged players to pick a stat-appropriate class and to alue these rare and powerful characters.
But how rare are they really?
Early D&D and its modern clone Labyrinth Lord give bonuses or penalties to actions based on ability scores, much like third edition D&D’s ability score modifiers. However, statistical analysis shows that bonuses are much rarer than in third edition. The “4d6 drop lowest” ability score generation method in D&D third edition gives a 61.65% chance for each ability score to have some bonus, and only a 17.51% chance of a penalty.
In Basic D&D and Labyrinth Lord when using 3d6, about 48% of ability scores will occur in the range 9-12, which in Basic D&D and LL is a +0 modifier. There’s a 26% chance of a bonus and 26% chance of a penalty, usually only +1 or -1.
Above-average ability scores also grant a bonus to XP if you choose a class for which that ability score is a prime requisite: 13-15 grants +5%, and 16+ grants +10%. Since class can be chosen based on ability scores, only one high score is necessary to gain the bonus.
Around 25% of characters qualify for the 10% XP bonus, and 83.48% gain some bonus XP, if there are classes available for all ability scores. Since the Labyrinth Lord core rules have no classes for which Constitution or Charisma are prime requisites, only 17.272% of characters will gain the 10% XP bonus and 69.893% will gain any XP bonus.
Here is the full list for Basic D&D / Labyrinth Lord. The Modifier listed applies to various uses of some ability scores, but not all. The percentage listed is the chance of this result on one roll of 3d6.
Score Mod. XP Percentage ----- ---- ---- ---------- 3 (-3) -10% 0.463% 4-5 (-2) -10% 4.167% 6-8 (-1) -5% 21.296% 9-12 ( 0) +0% 48.148% 13-15 (+1) +5% 21.296% 16-17 (+2) +10% 4.167% 18 (+3) +10% 0.463%