Well there are the obvious set rules such as the loss of both eyes causing blindness. Since you require at least one free hand to cast spells with a somatic component, someone who had his arms cut off wouldn't be able to cast such spells. You could arguably say the same for spells with material components since they must be manipulated in some fashion. Using your teeth or a foot to do that might not count.
I'd like to know for what reason are you asking this question? Are you a DM looking to perform some medically unneccessary amputations or a PC thinking of doing the same to some hapless NPCs? Either way, if you're thinking of performing such surgery during combat you'll be hardpressed to find rules to do so beyond things such as Torn Asunder since the combat system abstracts such effects such that they can only occur once the combatant is defeated or via DM fiat.