1) I knew there was something up with that platform, but never thought about jumping down from the side (tried a couple jumps across to stuff not close enough to reach).
6) In D2:ED I missed the hidden room when I went through the mine, and had to go back when someone mentioned it in the forum. I used the dragon stone to get out then, but in DKS I noticed you could still see the pressure plate on the wall after the outer door closed, so jumped up onto the table to reach it.
I had to look up where one of the parchment scrolls was in Broken Valley. I was actually exploring near the bandit camp and headed in the direction to find it, but got distracted by something and didn't go back.
I missed the malachite veins in the well cave (walked forward until I triggered the cutscene, reloaded to explore the right side, triggered the cutscene accidentally, reloaded to explore the left side, triggered the cutscene again by accident and didn't think it was worth it to reload for the bit I missed).
In the Lost Cavern I found the book and stood in front of the throne, but left too soon when I couldn't figure out what needed to be done (which was wait longer).
I missed the zeppelin part above Moor's cabin (only checked that ledge from the air and didn't see the nest under the tree).
...and probably a couple other things I don't recall.