You know this has come up on the world I used to play on a lot... And someone had what I thought was a good point. I'm just paraphrasing because I can't remember the original explanation and it was probably done better before but it went something like this:
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There seems to be a tendency in role playing to make our characters dumber than they need to be. It's kind of a strange concept but there should be a lot of things your character knows that you don't. They aren't blank slates, they're products of the world they live in, and in this world magic is a powerful and very real weapon. It may not be well understood by the commoners, but then adventurers aren't commoners.
I would think that in a world where magic is very real, and is very much used by the enemies (whoever the enemies may be), then the study of how it works would be pretty thorough. While I would never say characters ever use the D&D game mechanics terms, I would think they would have a means of figuring out what spells do what. Certain wizards or just scholars with a mathematical capacity might even try to express it in statistics, that is however statistics are expressed in that world. That's certainly valuable information to anyone who relies on spells to survive, or who expects to go up against a spellcaster isn't it?
(I would also think there would be those who understand how magic works who don't use it themselves, just like there are a lot of modern experts on guns who have never fired one. Even a warrior should understand some basics of spellcraft if they're expected to fight spellcasters. They may not know what spell is being cast when the caster is chanting and making motions, but they will learn to recognize the effects when they see the aftermath)
Of course there is going to be the unknown and there are going to be surprises, and lots of them, but at the same time when these characters walk into life-and-death situations trusting certain spells I would think they would have done extensive research beforehand on exactly what the spell is capable of. Just like when a modern day soldier goes to battle, he might not know what strategies and weapons or other tricks the enemy has waiting for him, but he's done all the research possible on what it might be.. And you can be sure he knows everything there is to know about the weapon HE'S carrying, and the defensive equipment he has at his disposal...
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Anyway take it with a grain of salt, but I'm not sure, say, knowing the DC save for Wail is metagaming because an adventurer who has a fundamental understanding of magic-- either as a caster, or as someone who's expected to fight casters-- would understand and even have a way of expressing their chance of success or failure. Obviously a wizard's understanding would be ever greater but even a magic-fearing barbarian would have some understanding of the tricks of the trade.
(Also note after all this that of course there are exceptions depending on backgrounds and all that, and what I'm talking about applies to relatively experienced adventurers, not first level fighters who just arrived in town)
