There are some exceptions. Vorpal weapons can sever heads. (I've got a nagging vague recollection of some other weapon that could sever limbs, but I can't pin it down.) In some editions, a Sword of Wounding caused its ongoing loss of HP by bleeding. The unnumbered-edition DMG notes that "If any creature reaches a state of -6 or greater negative points before being revived, this could indicate scarring or the loss of some member if you so choose. For example, a character struck by a fireball and then treated when at -9 might have horrible scar tissue on exposed areas of flesh — hands, arms, neck, face."