Nope, never tell the players AC, DCs, hit points, saves, etc. I make them wonder and worry. IME, throwing the numbers around completely destroys the suspension of disbelief for me and my group, and I have actually had to slap one player (who is no longer with us thankfully) with a hefty 50% XP penalty for ONLY speaking in terms of stats. He constantly asked the NPCs/monster's AC, HP, saves, DR, etc, and never said anything in character ("my character tries to intimidate the inkeep- what DC do I need?") ARGH! And he was surprised and outraged when he only got half the experience the rest of the group did. Usually my group never asks those kinds of questions though, and couldn't care less about trying to quantify their enemy's or NPC abilities.