DrunkonDuty
he/him
As a few other posters have pointed out up thread, there's a difference between the rules of the game and specific, in-game knowledge.
All players should have access to the all the rules that they want. Everyone should have the opportunity to know, for example, what fire resistance 30 means. Or what AC 20 means. Or what a twilight roll is (Ars Magica.) Or how to make a breakout roll (HERO.)
When it comes to in-game knowledge a player should be told anything that their character can reasonably know or observe. Including the game mechanical values for such. Both @Thomas Shey and @hawkeyefan give good examples of things characters can know just by observing. "That's a DC15 climb." "That ogre has AC18." Knowing this sort of thing is good. It speeds up play. Most importantly it lets all the players know what the hell they're doing.
Occasionally there are times when a "gotcha" can be good. When what the character observes/knows is in fact false. A gotcha can add a sudden twist to the drama. But twists MUST be occasional. And they can't be too out there. If the game is an endless series of gotchas with no connection between what the character can observe and what's actually happening... Well I've played in games like that and felt like I was hanging out in the GM's nervous breakdown. Not fun.
All players should have access to the all the rules that they want. Everyone should have the opportunity to know, for example, what fire resistance 30 means. Or what AC 20 means. Or what a twilight roll is (Ars Magica.) Or how to make a breakout roll (HERO.)
When it comes to in-game knowledge a player should be told anything that their character can reasonably know or observe. Including the game mechanical values for such. Both @Thomas Shey and @hawkeyefan give good examples of things characters can know just by observing. "That's a DC15 climb." "That ogre has AC18." Knowing this sort of thing is good. It speeds up play. Most importantly it lets all the players know what the hell they're doing.
Occasionally there are times when a "gotcha" can be good. When what the character observes/knows is in fact false. A gotcha can add a sudden twist to the drama. But twists MUST be occasional. And they can't be too out there. If the game is an endless series of gotchas with no connection between what the character can observe and what's actually happening... Well I've played in games like that and felt like I was hanging out in the GM's nervous breakdown. Not fun.