So if I'm understanding you, the DM is a jerk if he ignores the players' signaling when they want to use knowledge of weaknesses they have, but he's not a jerk if he ignores the players' signaling that they want to know the weaknesses of unique monsters. So now you've established that the DM isn't automatically a jerk if he ignores player signaling. Now I have to ask, though, where's the line drawn? Is there a list of what player signals the DM cannot ignore without being a jerk?
No, I think you're still missing a vital bit; the signal in the scenario I described is not "we want to know this monster's weakness" it's "we don't want to pretend we don't know this monster's weakness, please don't make us".
If the DM says "too bad, you all have to pretend you don't know about their fire vulnerability until something happens that makes me think 'okay you can use fire now'" then I think that DM is being a bit of a jerk. He's ignoring what the players want, and it isn't even over something vital or essential to the story....it's a minor factor in one encounter.
Is that clearer? It's not about the players trying to get an "unfair advantage", it's about the players being able to have input in some way on where the game goes and how it's played.
As for where the line is drawn, I would expect it would be different for everyone, given varying tastes and preferences.
No.
1. The DM is not the player, so he cannot be bringing in player knowledge to the character.
2. The DM is has the authority to grant boons like that.
3. Presumably Uncle Elmo is already a part of the PC's background and there's a reasonable change he knows about that.
However, I wouldn't do that, because a description is no substitute for the real thing, and it was a long time ago. Elmo might not have even met a troll, and even if Elmo did meet trolls and tell the PC about it, remembering isn't guaranteed, so the best I would do is give the player a roll to remember. And since it's something in doubt, I wouldn't bring it up as an idea the PC might have. That goes dangerously close to me playing the PC for the player and that's not something I would do. It would be his job to remember that Elmo might have said something and then make the roll.
Your answers make me think that this may be more about preserving the DM as the authority to introduce elements to the game, and also to decide when they can be introduced. A player cannot decide what his character may know in the world, but the DM either has to determine it, or grant it as a boon.
I think the way that sounds seems a bit problematic for some.
I realize you go on to say that you wouldn't run things that way, so I get that and appreciate it. But I think that the DM authority angle is what people are taking issue with. Again, I think a game can run perfectly fine with the DM having such authority. However, I think one of the challenges of such a game is for the DM to not overdo it. This is how railroads and Mother May I happen. Things to be cautious about when running such a game.
You don't seem to share such concern, so I think that's what's causing some of the conflict, as well.
As long as the DM is trying to be as fair and impartial as he can be, he's not going to outright say no about the trolls. The answer will either be yes, or roll the die. The pre-established background already had Elmo as an adventurer who went to the Temple of Elemental evil and survived.
I think this is a great solution, and one many have advocated for throughout the thread.