Caves. Everything is nondescript and murky, so you can't see half the passage entrances and even when you do, you've got so turned around trying to find it you can't tell if it's one you've been down before, then half of them don't even lead anywhere, or there are two or more that both lead to interesting places, but you don't know if one is just one room and then go onto the next one or going to lead further on so should you turn back and check the other one before you get too far? It just creates a feeling of frustration and missing out rather than a feeling of exploration.
That's a problem I've had in many games, but specific for Zelda ToTK which they changed since he older ones is, I'm fine with a bomb-able wall, but one and done is enough. It's bordering on farce the amount of earth you've got to break in this game. three or four rows of walls for a single passage or such a thick block the size of an entire chamber you go through several weapons or spend ages charging up the "reusable" option. One I destroyed 90% of the rocks and cleared the passage but there was another passage hidden behind the few I had left. Also don't feel like it's clear when to use ascend to get to secret chambers so I end up using it at random when I'm stuck. Also ultrahand-able rocks used as scenery then suddenly one is needed to be moved to get further. It's weird because they've done so well to subtle prod you in the right direection for s many other parts of the game