Hollow Knight drops you into a dark and mysterious world filled with anthropomorphic bugs, both friendly and hostile. Hearing that, you might be surprised to hear it is one of the most beautiful 2D worlds I have had the privilege of exploring. And that's what I want to talk to you about first - the world of Hollownest.
Exploration
Your journey begins in a small town of three or four buildings with an elderly bug warning you of the dangers that live below. As soon as you dive into the underground, your curiosity will take you through both dark and well-lit paths where you will have to dodge and fight a variety of enemies, from acid-dropping flies to proud warriors defending the husk of their old kingdom.
The mystery of the world is what drove my curiosity to keep exploring. To be completely honest, I had no idea what my purpose was at the beginning of the game. Early on you will find artifacts that seemed to be left behind by some ancient civilization, but nothing indicates what exactly you're supposed to be doing. Both in this game and games like Metroid, the environment does an excellent job of showing you how to progress in the game. For example, you will enter a room that seems to be a dead end and upon retracing your steps back, you find another pathway leading you to a powerup or item that will turn that dead end into a gateway to a whole new region to explore.
The thing is, with Hollow Knight versus Metroid, there isn't just some big baddy like Ridley to chase. Instead, what drove me to keep exploring was a desire to find out what the history and identity of Hollownest really was. The atmosphere is dark and lonely, sure, but there seems to be something bigger at hand. And that mystery is only further prodded along by all the interesting NPCs you encounter along the way.
NPCs
The NPCs are really a delight, and they really breathe a sense of reprieve into a game that is otherwise unforgiving in its difficulty. There will be times after a few fights or obstacles where you're low on health and frantically looking for a save point when all of a sudden you'll chance upon a hot spring and another lone traveler chilling in it. The hot spring will give you a chance to restore your health, but there won't necessarily be a chance to save (again, this game is unforgiving). But the sense of desperation makes these pleasant and surprising encounters with NPCs all the more refreshing. This is something that Metroid games lack for the most part, making this game truly unique.
That being said, the gameplay itself will definitely feel like Metroid. Even some of the powerups seem fairly similar to what Samus gets in her games. But in Hollow Knight, you're a swordsbug wielding your trusty "nail". That in itself is a big enough difference from Samus' beam cannon to know the game isn't a Metroid clone. If you're not familiar with 2D Metroid, expect precise platforming as you scale a vertical or horizontal area, fine tuned dodging to survive devastating boss attacks, and the satisfaction of coming back to an area with a new weapon or toy that will help you blast through enemies that seemed impossible to beat the last time you were there.
I hope this was enough for those interested in either the gameplay or story to get a sense of how Hollow Knight works. It really is a masterpiece in my book.
Difficulty
I do want to address difficulty. Expect to die countless times when going up against boss enemies. Fortunately, when you die, you keep any powerups or items you gained as you respawn at your last save point. However, you will lose ALL of your currency, and the game auto-saves as you die so you can't just turn it off to get your currency back. This can be frustrating, but wherever you die, you leave a "shade" of your past self. And you can can go back to where you died to fight that shade and regain your hard-earned cash. The shade will give you a little fight, but they are particularly easy to defeat.
That being said, the difficulty of the game is what creates the satisfaction of progressing to that next, beautiful region that you had no idea existed or earning that powerup that will open up all sorts of new exploration possibilities. And that simply creates a positive feedback loop where you're almost asking for more challenges, just to see what's next.
While I've had experience completing Metroid games, I would by no means call myself a platforming expert. The game was difficult for me, and I stopped for a bit at one point (or two or three), but whenever I got back into the world of Hollownest, I couldn't put the game down, even if it meant dying 10 times, then 20 times, etc....
Good luck and have fun :)