For a score of 15 or over:
STR: +1 to hit and +1 to damage if a Fighter
INT: +1 1st level m-u spell
WIS: +1 1st level cleric spell
DEX: +1 to AC, and +1 to move silently
CON: +1 HP per HD (same as a Fighter class gets, +2 if a Fighter)
CHA: +1 (positive) on reaction checks
HPs: Characters are only unconscious at 0 HPs. For each level a character may have a minus HP total equal to the level, so a 1st level PC is dead at -2, a 2nd level at -3, etc.