The DM can just handwave it when appropriate, too - but yeah, that's what I'm referring to with "barring in-game knowledge". If the opponents know the PC's or have experience fighting other's of their class, they could know much more than whatever the rules provide. That's a fun way to make it clear to the player's that they're dealing with smart opponents or that their PC's have been Noticed by the BBEG -- a roving band of NPC, a shout "it's them!", followed by a fight in which the NPC's exploit a few of their weaknesses or strategies...Can't the monsters just make a 'monster knowledge' check vs. the PCs?
There aren't the same effect.
Would a monster know of a ranger's Disruptive Strike power before he uses it?
They certainly don't support automatic knowledge of what a power does to others either
But for the various reasons previously discussed (in short, RAW and powers that don't work otherwise)
You're suggesting that the rule is vague and difficult to execute. Well, (A) it's D&D, there's a DM for a reason, use common sense.
Your point being? It's a difference. Due to the phrasing of the rule which says that creatures know what been done to them, it matters. The difference between fire damage and psychic damage doesn't matter either; it's the same effect - right? But it still can matter when the rules make the distinction; and they do here.There is a semantic difference, and only that.
Yeah, the number of times this will seriously matter will be... vanishingly small. The only standard mechanic would seem to be marking, but even there it's good practice to keep things varied (i.e. sometimes violate and sometimes not), and if you're doing that it's not like it's very relevant whether the monster should-have-known and wasn't the smartest, or couldn't have known but got lucky, or whatever. So actually I think it's not the best example.Eh, for clarity... as long as the group knows how it works, I think you're fine going with whichever way.
Heck, part of my interest in this is that it's very rare that Eamon and I disagree on anything![]()
And Brash Strike... well, if the monster knows the details of the power, it's totally pointless, but it's not exactly recommended even if it doesn't. And if a player were to pick such an obviously borked power, it'd be nice to run it in a way that does work, regardless of the rules.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.