One of the rules I regularly ignore in AD&D is the notion of "system shock survival". This is a rule that states, essentially, when one's physical system undergoes a drastic shock (such as when the character is polymorphed, turned to stone and back again, etc.), one must make a roll in order to survive the transformation. The number you have to roll is based on your constitution score (CON 3 = 35% survival, CON 18 = 99% survival, with 70% being about the median).
I really hate this rule because it seems to me to be a sort of "anti saving throw". Unlike level draining undead, which I see as one element in the DM's arsenal to force the players into making logistical choices with real consequences (and which can be countered either by thoughtful tactics or the spell restoration), the system shock survival roll seems to me to be a second chance for the DM to kill off a character. I'm not sure it's a necessary thing,
It also, I should add, turns something like the polymorph other spell, already a pretty potent bit of magic, into a turn-you-into-a-guinea-pig-in-between-two-chances-to-die sandwich. Think about it; if the base chance for an average (CON=10) character is 70%, you need to make two such rolls (70% x 70% = 49%) in order to survive the experience. That's ever so slightly less than 50%. Yikes!