Having to talk to all the characters in all the locations about each and every inventory item. Then, as soon as an new thing is found, to have to do it all over again. It's the main reason I prefer games with solitary exploration.
I WANT to show every inventory item to every character. That's something I liked about the first Broken Sword game, and missed in the second. In the first you could show just about every item to every character and get a funny response.
I find first person games with puzzle after puzzle after puzzle and no characters to talk to extremly tedious. I'm not a Myst fan.