Role playing games are games of conflict. The players want to take the dragon’s gold, or want to unseat the current leader of a town, or want to sneak into a room unobserved. They are always trying to do things that one or more entities don’t actually want them to do.
Because it is so common, a lot of the feel of a role playing game comes from its conflict resolution mechanic. At my current level of research, I know of two major categories of conflict resolution mechanics: dice and cards.
Dice are the classic conflict resolution mechanic. You roll one or more dice and may or may not add a number to that result. You are trying to meet or exceed a target number that would indicate success (often called “difficulty class,” which is what I’ll be calling it here).
In D&D 5e and Pathfinder, the die that you are rolling is almost universally going to be a 20-sided die and you will add numbers that indicate your proficiency. For instance, someone that was merely average at sneaking around might get to roll a 20-sided die and add nothing to that roll to see how well they sneak. Their range of possible values is from 1 to 20. Someone that’s quite good at sneaking might get to add 9 to that value, making their range 10 to 29. An untrained person could still beat them on their best day, but the trained person is going to be better the vast majority of the time. Every number in your range is equally likely.
In Cortex Plus, you roll two (or more) dice, keep the highest two, and add nothing to their results. Your proficiency at a task is represented by the size and number of dice that you get to roll. Someone that’s bad at sneaking might only get to roll two 4-sided dice, making their range 2 through 8. Someone really good at it might get to roll three 10-sided dice and two 6-sided dice. Since we’re only keeping the top two, the range becomes 2 through 20 for the highly trained person. Interestingly, the bottom of the range doesn’t change at all since we aren’t adding any numbers to our results, but the highest end of the range gets noticeably higher. Also interestingly, all results are no longer equally likely. If we’re rolling only two dice, our results start looking like a bell curve. If we’re rolling more than two, the peak of that curve starts getting pushed higher and higher. While the worst-case scenario for trained and untrained remains the same, the average case definitely changes drastically.
I have recently been playing Fifth Age (an old RPG, but one that breaks enough expectations that it’s worth playing just for the experience). In that game, you have a hand of cards. These cards have numbers and suits on them similar to your average old playing cards. When you would make a roll in a more traditional RPG, you instead play a card from your hand. The number of the card you play substitutes for the dice roll. You do get to add a number to this result. After you play this card from your hand, you get to draw from the deck so that the number of cards in your hand stays (mostly) consistent throughout the game.
Thusfar, this system has an interesting result — if I know what’s in my hand and I know what the difficulty class of the challenge at hand is (which is sometimes, but not always true) I can know for certain if I will succeed or fail. This does on the surface take some of the drama out of the game. Interestingly, most of the time when I know the difficulty class in Fifth Age, I am doing something that I wouldn’t normally have to make a roll for in more traditional RPGs, like casting a spell on a willing target.
Fifth Age does add one more interesting twist though when it comes to the suits of cards. Depending on what you are doing, there will be a suit that is considered “trump.” If I was trying to attack someone, for instance, then a sword would be considered “trump.” If the card I played from my hand in order to try and attack was the sword suit, I would get an additional card taken from the top of the deck added to my result. This means that I can willingly introduce more randomness into the result than normal. If I know that the difficulty class is 15, I can choose to use the 10 card in my hand (which is valuable always) to guarantee success or the 2 card that happens to be the right suit and hope that the top card of the deck is a high enough value to push me over the edge. That would allow me to get rid of a not-very-useful-card and replace it with a random draw. I find that adds an element to the game that I do enjoy.