What I mean by "Zelda-like items" is that it's something that's not just a key, but a new toy, a new game mechanic. Like Missiles, which become a backbone of the combat aside from their initial purpose of opening red doors, compared to the item that opens blue doors in SotN just by having it in your inventory, or the mist form that passes through grates so you can get into a few rooms but is of very little use otherwise. When other Metroidvanias have something like this it's usually limited to the basics like double jumps, slides, and dashes. Metroid not only has way more items than most others (the obvious example is how Hollow Knight is like five times longer than Super Metroid but has half as many important items at best), they all have to serve some sort of double function like this.
As for level design, it depends on the game how far it leans into it obviously, but something like the infamous room early on in Dread where you have to shoot out the ceiling strikes me as a distinctly Metroid thing that other games don't usually do. You have breakable walls in Castlevania, but it's purely for secret rooms.
