Motivation isn't the issue. At-table results are.
Yes, EXACTLY. All that matters is what happens at the table.
So if you're playing with your 6 year old niece, and you know she's never played D&D before and knows nothing about it, and she just happens to do the right thing (fire on trolls or whatever) do you get bent out of shape worrying about her motivation, or do you think that's AWESOME and cheer and have a great story to tell the next time you see your group?
So what's different about the veteran player doing the same thing.
The difference is entirely in your head. There is no difference in what happens at the table, but in one case you are getting upset about what you imagine is in somebody else's head. So don't do that. (Or, I mean, do. That's up to you. I'm just illustrating why your stance is about you, not about the other player.)
Which simply forces the work over to the DM side, to make those changes; and after several campaigns it's only natural she's going to run out of new ideas.
Umm....yes, good DMing takes work. This is kind of exactly Angry DM's point: metagaming is the fault of lazy DMs.
Big difference; in that when trying to fly you're interacting with the setting's physics and thus open to game mechanics and-or DM rulings; while when just "knowing" something intrinsically you're not.
No, in both cases it's up to the DM to decide if it's true. Anything the player believes to be true can be not true, at the whim of the DM.
If I (the other player) have to roll to determine if my PC has knowledge that you've arbitrarily decided your PC has, that seems off somehow.
I'm happy to keep repeating this, but if my PC has decided he/she knows something, that doesn't make it true. The knowledge skill is used to determine whether or not something actually is true.
Question: What's the difference between a player who decides their character knows that trolls are vulnerable to fire, and the player who decides that the combination to the safe they just found is 38-17-45?
Answer: Absolutely nothing. In both cases the DM decides whether or not what the player believes is actually true.