The vast majority of rpgs, on the other hand, devote most of their rules to those things that are considered extraordinary, but within the bounds of the setting. (For a wonderful discussion of the quantity of rules in a game, and what it means and doesn't mean, check out this post over at How To Start a Revolution....) This would include (to taste): martial combat like swinging swords and shooting arrows, magic in nearly any form, character health and performance, character death and destruction, nuanced or uncommon skills like riding a horse into battle or singing a beautiful song, avoiding bad things be they fireballs or spiked pits, navigating magical or intensely wild landscapes, etc..
Then there are my favorite things, the Feats of an Unorthodox Nature. When designing a game, you have to decide whether this category of actions will be a very long list, or a rarely referenced, highly obscure group of things that no one ever does. A perfect example, last night I was flipping through my friend Max's 5e dmg, and I came across an optional rule at the back: climbing on larger combatants. In our 3.5 games, this was unheard of. No one ever even considered it a viable option. In 4e, I seem to recall one Rogue class power that could be taken that allowed for climbing combatants. No one was going to play a rogue simply to have that power at level 17 or some-such, however.
Now here we are in 5e, with an optional rule that would allow anyone to attempt this radical maneuver.
Personally, I don't care one way or the other whether you allow climbing opponents in your game. But the way you rule it in or out affects how the players will use it.
Think about it. You sit the players down for the first 5e session and declare the "climbing combatants" rule is fair game...and suddenly it's climbing season. You won't be able to get your players off the big boss monsters. And why not? We're all here to have fun! Why wouldn't we use the rules that sound the most fun?
But you don't want your players climbing your monsters constantly! You want them climbing monsters when it's cool, and fits with the flow of the encounter, and when they aren't just trying it for the attack or defense bonus it grants.
Here's how I fix it. Don't write a bunch of rules for every cool but crazy thing a player could do, just write one good rule for this category of unorthodox actions. My rule is:
- Want to do something crazy awesome? (Or maybe just crazy?) Roll the check with Disadvantage. Success works as written, but failure has two states: If a single die is too low (i.e. you failed due to disadvantage) then you fail. However, if both dice results are too low, then you fumble.
I was inspired by the 5e rules, and by this Called Shot Mechanic by Zak S. Later, I actually found the same exact rule over at The Last Gasp, but without the advantage/disadvantage terminology.
This fixes the problem of non-use and abuse of crazy maneuvers by giving the players a "blank check." You aren't pushing the players (unconsciously) toward a particular choice of action, but you are also opening the door for narratively-appropriate inspiration.
If that troll is begging to be climbed on, then the players will try it. But if there's something even cooler burning a hole in your player's imagination, they'll try that instead. That's the difference between a rule that provides players agency, and a rule that provides players with a script.