Skill-system-free progression in Brogue
Brogue is probably my favorite traditional roguelike. I've played several others, chiefly Dungeon Crawl Stone Soup, Tales of Maj'eyal (ToME), Ancient Domains of Mystery (ADOM), NetHack, Angband, Cogmind, and probably more I've forgotten. None of them is quite the same as Brogue.
In some roguelikes, there is a skill system. You have a list or a tree of skills that affect how good you are at different tasks. Increasing your skills gives you benefits such as new abilities or numerical bonuses. New abilities might be for example wielding new weapons, crafting new items, or shield bashing enemies. Numerical bonuses include things like hitting enemies more often when you attack using any type of spear. By taking action in the world you can gain skill points, either directly by performing the skill to be leveled or indirectly by gaining XP and investing that in your skills. In Dungeon Crawl: Stone Soup (aka Crawl or DCSS), you can open a menu and choose what skills you want to improve; when you gain XP your skills directly gain some (usually fractional) number of skill points. In Tales of Maj'eyal (ToME), you level up through XP and leveling up gives you points to invest in your skills as you choose.
The thing about skills is that they build on each other. They're investments. Skills require you to do A before B. To reach level 15 you have to first spend a bunch of points to get to level 14. To unlock Shield Wall you must first invest enough points in Shield Bash which comes before it. This means that if you want to have the right abilities to come online for you at the right part of the game, you have to plan and manage your skills.
This is frankly something I'd rather not do. Frequently the A before B structure is not explicit or is hard to find in-game. (I'm looking at you, Crawl of six years ago.) But even when it is, the details are generally larger than my working memory and I don't care to memorize it. And also it means that in Crawl for example, I have to every so often remember that skills exist and pause to consider if I'm still headed in the right direction in the context of my dungeon-delving options. That pausing and re-planning is something of a pain.
Brogue is unusual in not having a skill system. In Brogue your progression instead focuses on what items you have. (If I recall correctly there are one or two other sources of progression, but they are more niche.) It's your inventory, not your character, that determines what you can do. Your best items are limited-use: Darts, potions, scrolls, wands, and staves. Staves have a limited, small (2-5?) number of uses, but will recharge slowly over time (if I recall correctly). Wands can only be used 2-7 times before they fall apart, and unless you identify them you don't know exactly how many uses you have. Darts, potions, and scrolls are strict single-use items (though darts come in multi-use stacks).
Because of this, when you encounter something dangerous in Brogue you focus on your items as your options. You could use that wand of disintegration tucked away and melt that ogre. But you may yet meet more dangerous things; is it worth it? You've got that potion of fire; maybe you could burn the cave moss and kill or injure the ogre that way. You could drink your levitation potion and escape across the chasm. You have decent health; you could forego the items and risk combat. These options combine in interesting ways, but the key question is: What item are you willing to spend to deal with this?
The key thing I think is: You don't have to think about skills. You rarely need to explicitly remember to stop and think; items and encounters are perfect prompts. You don't need to think about skills you could take or which you want to have in a few levels. You don't have to remember that other set of options you have that are buried in that other menu. In any given situation, only the items matter. All that matters is: What resources (items, health, space, positioning, visibility, map) can you gain? What would it cost to get those?