Swordfighting in the high grass.
In a fast-paced fighting game where understanding the state of your opponent is important, anything that inhibits that understanding changes how you play the game. Nidhogg’s beautifully deforming tall grass does that in a very interesting, mechanical way.
The grass has three states: still (where it is actually moving very slightly, but opaque over all the body area but the head), disturbed (where it is still upright but allows visibility via gaps), and flattened (where the view of the characters is entirely undisturbed). The game switches between these states based on the motion of the players through the space.