Off the top of my head, just in New Sorpigal there are the scroll of Fly hidden in the south wall of the bank, the chest hidden in a rock on the coast, the buoy that sets off the volcano, the teleport to Dragonsand on top of Buccaneers' Lair, and there may be one of two more that I can't lay mind to right at the moment. Consider just the Fly scroll – somebody's probably been round and clicked on every wall in town to find that. The amount of effort and ingenuity that it has taken to locate what has been found is staggering.
And I really do take my hat off to whoever it was that found the features in the v1.0 game which enable archers and the like to learn the mirrored path magics. I would guess the critical spots can't involve more than a double handful of pixels.