AC is shorthand*. It's actual definition "Armor Class" is even an misnomer, as you can have a high AC without wearing actual armor at all!
I could say "he's wearing no visible armor, but you can see the telltale signs of an Abjuration effect, likely Mage Armor, carries a shield, and is remarkably swift"...or I could say "he has AC 19" and it means pretty much the same thing.
Arguably, one could attempt to dispel a Mage Armor where you can't a suit of physical armor, and that's a consideration that could be made, if I ever saw anyone actually do it (outside of the high-level 3.5 Dispel meta). And as for Dex bonus, well heck, you don't even lose that anymore, so it's also practically no different from physical armor (unless you need an indication of whether or not a target has a high Dex save, and you could just say that if someone asks).
*I realize this isn't a D&D specific thread, but many systems have a defense rating of some kind, and even those that don't have some other form of defense that makes you less likely to suffer harm in combat.
If someone doesn't want to use shorthand, and wants to describe every opponent the party faces (and trusts that their players will grok what is being said), that's perfectly fine. I, however, have found that using the game's shorthand and cutting out the middleman of my brain trying to describe it, and my player's brains trying to decipher my statements, makes for a smoother play experience.
By contrast, my current DM never announces AC, but will tell us if we're close to hitting someone (so that we can use Bard dice, rerolls, or Luck points* as necessary). And by round 2, we generally know what we need to hit anyways. If he just said "this guy has an AC of 15", I'm not sure much would change, but it's his style and I'm fine with it.
*Kobold Press optional rule in use for his Midgard campaign.
You don't have to give out statblocks, and to be honest, most players I know can't handle that cognitive load, lol. If I read off the whole stat block, I'd get the same glassy eyed stare in response as I do to my flavor text! This isn't a knock on their intelligence, just that some people like to turn off their "work brains" to best enjoy D&D. A guy I play with regularly is an EMT who absolutely knows how to do math and memorize lots of information, as it's vital to saving lives- but when he plays, he'll have trouble calculating his damage rolls, even when he uses the same pool of dice and modifiers a large percentage of the time!
And it's still possible for enemies to have defensive abilities that can modify their AC "oh he can parry/Shield/what have you", or keep hidden certain nasty special abilities, if "surprising" your players is important to your play style.
Now let me address another comment I've seen several times so far- giving the players information "for free". The DM is the only source of sensory input for the players. They see what you tell them they see, they hear what you tell them to hear. I don't know about other people, but no description, no matter how detailed, can substitute for what their characters could conceivably sense- ie, a picture is worth a thousand words.
Plus, in the heat of the moment, you might forget a detail that the players would have otherwise picked up on. It happens- I once infamously mispronounced a word I had only ever read while reading a text block in an old 1e adventure, and thus my players missed a critical detail about an encounter! So in my opinion, it's very likely that much of the time, players are getting less information than they would if they themselves (let alone their characters, who are likely superior to them in many ways) were in this situation.
I'd rather err on the side of giving them more than letting them walk into a "gotcha" moment that shouldn't have been one, but even so, there shouldn't need to be a "special ability" to let someone know whether it's a good idea to engage a situation or flee from it (as fleeing gets harder the longer you've engaged in a combat).
Unless, of course, your group's preferred style of play is uber-cautious, combat is the very last resort, we poke at everything with 11' poles routinely send waves of hirelings to their doom, and every single thing you see could theoretically be a deadly monster in disguise.
