My response is to ask you "why assume a mystery would be more entertaining or tactically challenging?".
Oh, I don't know. Maybe because knowing all kinds of information about one's opponents so that your tactical decision making is easy, is boring. How many players go "Oh Crap!" when hit by a breath weapon when they know the foe has it as compared to when they get surprised by that in combat? Which is more interesting and entertaining? In both cases, the PC got hit by the attack. In one case, it was a surprise and more fun for the entire group. In the other case, it's just another attack because the players know ahead of time that it's coming.
Discovering things and overcoming challenges (including the challenge of discovering things about your foes) is generally more interesting than the DM just blurting it all out in round one.
Do you find placing hidden traps in the encounter more or less interesting and entertaining than having traps lying around with the word "trap" on them.
I really think this concept of the player being heavily informed is a type of "WotC entitlement brainwashing". Somewhere along the line, someone there discovered that some players feel entitled to all kinds of things, so they wrote the rules and guidelines that way in order to sell more product.
This is an excellent marketing ploy to keep players happy and wanting more, but it's not how the game should necessarily work. The PCs should not be entitled, the PCs should earn what they gain. Blurting most of the info about foes out (which is no different than auto-identifying items during a short rest) has become the new mantra of "well, that's how it should be played". A good portion of the D&D gaming community has bought into this hook, line, and sinker.
It's like someone mentioned earlier in this thread. It's like candy. Keep handing out more and more candy to the players and they feel like they are entitled to candy every time.
An interesting anecdote to this. I had a special adventure for one of my players. She was trying to find a minor relic for her goddess and the actual side adventure for it consisted of 2 n+2 encounters and 1 n+4 encounter. I had no treasure parcels in the first two encounters, but had 4 treasure parcels in the final encounter. After the second encounter, one of the players said "Hey, no treasure for either of these fights. What's up with that?". Players are so used to acquiring treasure parcels most encounters that two in a row without was jarring enough for the player to comment on it. Now, he was probably kidding, but they do feel entitled to it anymore.
A DM can always obscure the minion status of an opponent, forcing players to discover their minion-ness through trial-and-error. Or through mechanism like a skill check. But neither of those result in particularly deep or clever play.
It's not the point to discover minion-ness. It will happen eventually, but that is not necessarily the goal.
The goal is to defeat the opponents.
Discovering minion-ness is no different than discovering elite-ness, or vulnerabilities or special attacks or anything else about the foe.
On the other hand, a well-designed encounter employing 'transparent' minions presents the player with more interesting tactical choices, ie clear the minions or go after the 'stronger' targets, questions which depend more on the current battlefield situation, change over time, and are therefore more complex.
Interesting? You mean like chess where you know which pieces are the pawns?
How is giving that information out interesting in any way?
It's actually less complex to give out minion status and makes the tactical choices a lot easier.
Your logic is backwards here. The Fighter doesn't make a tactical choice with regard to minions, he makes a no brainer choice. If he has a burst one power than can defeat a bunch of minions and he has the opportunity, he typically uses it. If he doesn't, he attacks a non-minion so that he doesn't waste damage.
That's like saying that paying taxes is a choice. Yes, it is a choice to either pay taxes or go to jail, but the choice is already made for 99% of people. They don't really make a choice, they just auto-select the proper choice.
Sorry, but transparent minionness results in fewer and less interesting tactical choices. The choices are limited by definition.
In the "hidden minion" game, the main tactical consideration w/r/t Minions is "don't accidentally waste a Daily on one". You could argue more complex tactical decisions depend on the ability to distinguish Minions from non-Minions. Sometimes giving the players more information rather than less makes for more tactically-rich encounters.
Most people typically wouldn't waste a daily on standards or elites either. The threat has to be perceived as serious before dailies are typically used. By telling the players minion status, you tell them "Oh yeah, there are 15 foes here, but it's not really a threat. Don't bother using your Dailies".
The DM for all intents and purposes auto-limits the choices of the players.
When the players do not know, they have to feel their way through the minefield and make their own choices including choices to do actions that may or may not find out if any of the foes are minions.
I'm not saying disguising minion status doesn't have its place. But to say it automatically adds some sort of beneficial mystery to the game is too simple.
It adds more of a mystery than just blurting minion status out.