CHANCES TO HIT AN ENEMY OR MONSTER: In Ad&d you have better chances to hit (THAC0) if you are a fighter than, say, a mage.
In Powers & Perils: You have better chances to hit regardless of the concept of class- rather, if you have high scores in strenght, stamina (to endure the prolonged fight) you'll end up having better chances. Also, your combat experience level plays a role on this aspect.
HIT POINTS: In Ad&d if you are a fighter you roll for more hit points. Instead, if you are a mage, you are doomed to have less hit points.
In Powers & Perils: How many hit points you have depend on a formula which takes into account your strenght, your stamina and your constitution.
Besides:
In Powers & Perils each time you successfully use a weapon against an opponent you earn experience with that weapon, thus making you more skilled with it (but you have to score at least a certain amount of damage during that encounter in order to have this gain).
EXPERIENCE POINTS: In Powers & Perils when you earn experience points and you reach a specified amount, you can raise some of your characteristics (those that you logically used, such as strenght, stamina, dexterity, will, etc..)
Consequently, if your scores get better, even your offensive combat level (OCV) can get better (thus having more possibilities in the future to hit enemies), and you could get more HPV (hit points value), because as specified above, they depend on your characteristics.
Ad&d doesn't take this aspect into account, you are supposed to never raise your characteristic scores. You were born that way, with no chance to improve your physiology.
Moreover:
COMBAT: In Ad&d, almost every weapon does different damages, if you need a 13 to hit a monster and you score a 18 it's the same. In both cases you hit him, without any significant difference.
In Powers & Perils, all of the weapons do the same damage, BUT the big difference lie in how "high" is your roll. If you need a 78 to hit but you roll a 30 with the 100 sided die, you are likely to score a SEVERE or even a DEADLY hit.
A sever hit yelds more damage than a normal hit (in this case, for instance, you are entitled to roll more dice when you roll for the damage. You could kill a giant if you are incredibly lucky (remember Frodo?).
In Powers & Perils you can try to use your round to dodge the enemy blow, and there is a dedicated table for figuring this out. Basically, your chances depend on your dexterity and on your agility, compared to the DEX and AG of your opponent plus the kind of armor you are wearing.
These are just a bunch of main diference, but there are several others. I think i'm going to start a series in the future entitled "Powers & Perils" lessons. It might be useful for another purpose as well, that is, to debunk a false myth concerning the unplayability of P&P due to its alleged complexity.
P&P books are full of tables Richard Snider wrote that gives immediate results readily available for the DM and the players, so that you are not obliged to calculate them on your own losing time and making mental effort.
But if i am playing Powers & Perils and i am enjoying it who am NOT an english mothertongue speaker, how much more easily that will be for many of you out there?
Last but not least, there is a final consideration that has to be made: it seems quite difficult to me to go back to AD&D nowadays, and i heard others have experienced this sort of feeling after playing Rolemaster and other similar games. I would not be able to stand the illogicity and "silliness" of many aspects of AD&D, how it handles things. I played P&P for the first time several years ago, i was not hooked with it because i was young and i thought that AD&D was the fantasy RPG, stop.
No need to wonder, no need to explore. Now that i decided to go back to complex rpg's, i found out what i missed.
