Lanefan
Victoria Rules
And there's a valid question to be asked as to whether this trend has been a good or bad thing; and the answer is probably some version of "both" for most of us.And this idea, of the player keeping keeping hp scores secret, was a widely-discussed technique around 40 years ago. But I don't think it's much in vogue anymore.
Exactly. They might not know a thing about what they're potentially going to face but they know, or have a vague idea at least, what resources are available to them before going out to face it. And even then if they decide to run out a party consisting of nothing but thieves I ain't gonna stop 'em, and they'll learn in-character where their shortfalls lie.But that generates my next question: these PCs know enough about what "dungeoneering" is to understand what a proper "combined arms" force looks like; but are completely ignorant of trolls' vulnerability to fire.

Not necessarily.They know that dungeons have traps that the "sneaky-types" might spot and disarm; they know that dungeons have monsters who will hurt them, thus generating a need for healing; but they don't know which of those monsters is vulnerable to what sort of damage the wizard might do.
They can surmise that it's likely there will be times when quiet scouting or infiltration will be more useful than a frontal charge; they can surmise that there's likely going to be times when having some muscle around would be helpful (even if only to carry out all the valuable loot we're gonna find!); and while they might not appreciate the need for a healer right away they probably will after their first foray into the wild.
As for the wizard - no idea what use he's gonna be but he's keen, so might as well haul him along with us.

Not as arbitrary as you want to make it appear, I don't think.That's a very arbitrary set of stipulations about "common knowledge".