You're looking at a group of soldiers. Eight of them are armed with standard issue AK-47s. One has a rocket launcher. One has a set of communication gear. One has a hat with the insignia of a lieutenant. One is sitting in a tank.
Which are the privates, and which are the noncoms and officers?
'These guys obviously have tricks these other guys do not' is NOT a metagame argument, it's common sense.
Especially in a game based on the concept of being able to take on armies of these guys and defeat them in one go.
Batman doesn't go 'Oh, these guys must all be supervillains' when he encounters normal mooks. The Bride doesn't flinch when she's taking on the Crazy 88. Heroes don't look at the rank and file and go 'Hey guys, we're out numbered by guys who are probably better than us.' Maybe after the first battle, but after the tenth battle like that?
And that's not even 'once they hit paragon tier.'
Your argument about doesn't support the proposition that "In-character differentiators(IC) (equipment, posture, other perceivable clues) should allow players to distinguish out-of-character(OOC) (or metagame, if you prefer) designations."
You talk about being able to distinguish privates from non-coms based on equipment, for instance. What this allows you to do is distinguish
IC roles or designations, not out-of-character ones. Just because a guy is "armed with [a] standard issue AK-47" doesn't mean he is not a poor, great, or exceptional combatant. You may be able to tell that he is a private, an in-character role, based on his loadout, but it does not necessarily follow that he is a minion, standard, etc. enemy.
It is entirely possible the private's only experience with the weapon is the week of basic training he got when he was drafted (likely a minion). Then again, maybe he grew up on an isolated farm and has years of experience in using firearms to kill predators or vermin (likely a standard). You may not be able to tell that from a quick inspection of equipment and stance before combat, and even if you can discern their background in such a way, it just tells about in-character abilities, not game designations.
I view minions as enemies that pose a minor threat to a PC. This does not mean that they have to have poor equipment, poor health, or poor posture. A minion may just be exceptionally unlucky, or a swordsman with a limited repertoire of moves. An imperial palace guard may be impeccably outfitted, have a good combat stance, moral, and carriage, be more experienced than the PCs (higher level),
and still fall to a single strike from a PC if he's a minion. The term "minion" is an OOC designator that has no affect on his in-character portrayal. It just means that he individually wasn't an important contributor mechanically to the combat. Fluff that how you want.
Note, even though designators like "elite" or "minion" don't affect IC portrayal, doesn't mean they
mustn't. If you wish to portray minions as inferior looking, I don't think anyone will say you may not, but you don't
have to.
As for telling players if monsters are minions, I may if they directly ask, but my goal as DM is to involve them so much in the story they are acting out that they would rather act on IC portrayals of the game world than OOC ones.