I feel as if I might have addressed this topic online before, perhaps on Google+, but since I can find no record of it (and Google+ is dead), I am returning to it here. As a referee of whatever edition or reinterpretation of D&D you prefer, do you declare the armor class of the monsters (or non-player characters) that the player characters face, or do you let the players figure it out for themselves and just tell them when their rolls miss or hit?
My method is this: When the player characters first encounter a foe, I describe the foe. The players can assess the level of threat posed by the foe based on what they know from previous experience, research, rumor, and observation, but until they actually attack that foe they can only estimate what the armor class might be. Once they declare an attack, I tell them the armor class. Because why wouldn't I?
I'm sure there are those who argue that stating the armor class subverts immersion, but to them I say forcing the players to keep track of their rolls each round to calculate the enemy's armor class subverts immersion more. If you are that concerned about the danger to immersion caused by stating the armor class, why don't you remove the danger entirely by simply making the players' attack rolls for them and letting them know whether they hit? It is not unknown for some referees to use this technique. They say it's all in the service of immersion. I think it deprives players of some of the thrill of playing the game. When the referee rolls for the players and never rolls in the open, how is anyone to know that any roll is ever fair, and how is anyone able to gauge what the odds are of doing anything? Anything or everything could be the result of DM/GM fiat, and at that point are you even playing a game or are you just taking a virtual tour?
I like my players to have an idea of the risk involved. It adds tension. It adds drama. It increases the psychological reward. It lets them know that they are playing the game as active participants and are not merely pawns of the referee. It doesn't "break" the game to let the players know an opponent's armor class. If anything, it lends the players a bit of confidence that the referee is fair, and that's a good thing.