The mechanic is how weapons aren't ever clear upgrades. Enemies have different weaknesses and while you can level up your character, the extra health or ability isn't ever remotely enough to trivialize an encounter. The game forces you to learn how to handle various situations.
In most games (at least all RPGs) the enemies get stronger as the game progresses, but so do you, and the gameplay never ends up changing. In Horizon almost every time I came across a new machine the same pattern happened:
1) How the hell am I ever supposed to stop this thing?
2) Oh okay it has specific weaknesses, how the hell do I exploit them correctly?
3) Oh, that's tricky but not impossible
4) Think I'm getting the hang of this
*after a few solid kills
5) Oh I know how to handle these now!
But then you come across a new enemy, and it starts all over.
I've been playing games heavily since I got Mario on the NES in 1989 and I can't think of a game that more strongly gives the feeling of progression, not by artificially strengthening your character, but by forcing you to learn how to respond to different threats. In just about every game I can think of, you learn to play it and the same strats work from start to finish, I've never seen such dynamic gameplay. Am I just intoxicated and blinded by the beauty and story and characters and all that's great about this game? Or did anyone else have this experience regarding the gameplay?
tl;dr: The game forces you to learn different encounters, not just get good at one strategy and use it from start to finish.