EXACTLY.

People in this thread keep saying that they should be expected to keep the information that they as a player have because it's obvious. No one is really giving an argument other than the fact that it hurts my fun. No one else who is making that argument is thinking about how the fact that that hurts other players fun because it makes it all about them.

You are hitting the nail on the head. You are able to have your cake and eat it too if you combine the roleplay with learning the information. You know you're going to a place called Troll Canyon? Go do the research. Suddenly you now do know that they are weak to fire. No one can argue that fact, and everyone can prepare. Moreover, you're also going to be in an area where you could probably get some extra fire stuff to help take care of them. It's also the type of cleverness that a DM will actually reward instead of just going. Oh yeah, of course you would know the thing for no reason.