General Overview
Shadowdark is often called a trimmed down 5e with OSR sensibilities. I think that is wrong, I do think it has a lot of the sensibilities that are broadly called OSR, but I don't really think its a trimmed down 5e clone simply because it uses a d20. It is clearly its own thing and it has a coherent vision of how it wants play at the actual table to flow, which I think is a huge improvement over most released games. It was the 2024 product of the year for a reason.
A Mechanic that works really well
I am going to parrot a familiar talking point and say the way Shadowdark handles light. How light is handled in a game sets the tone and gameplay expectations of danger. By the time AD&D 2e was in its final moments light had been relegated to an annoying detail that would ideally be handwaved if not for the requirements of verisimilitude. Most of the average adventuring party had some form of infra or ultravision and spells to grant continual light were pretty common for any adventuring party after the third level. Likewise, in the game, hazards were treated as mostly narrative obstacles with a growing focus on drama and story.
Later D&D it formalized more as a "gate" to be passed with levels and abilities, much like needing magic weapons to fight certain monsters. You just waited until you got the key then ignored that type of problem for the rest of the campaign.
OSR games treat it as a resource to be managed in the goal for treasure. Light is a timer for dungeon turns spent searching. You can take more torches with you, that lowers space for other gear and treasure. Its the rocket fuel equation.
NGR (because of course I am going to relate this to my own works, I am only human) has light be a dial between the odds of encountering danger and how dangerous it is should you encounter it. More light makes random encounters happen faster but lower light gives increasingly dangerous penalties for if you run into something. NGR is about choices like that.
Shadowdark has light last for a specific amount of real world time. It does have inventory management, but the real world timing makes it more accurate to say that there is an in-game penalty to wasting the very real and precious time that a handful of real people have managed to claw out of their busy adult lives every week. It is a ticking clock, a strategic version of classic Space Hulk's tactical egg timer. It fights Analysis Paralysis and distracting side conversations. It is a rule emphasizing that this is a game to be played by real people and not just game theory.
A detail that lifts above its weight
The class titles build in a lot of world building in a small amount of text. You can tell about what level various NPC's are, their likely alignment, and class. The alignment categories also have a pretty good hinting at alignment as cultural values rather than a modern concept of moral values (there is no good or evil, only law and chaos).
Constructive Feedback
The part of my brain that likes to tinker initially wanted to make comments about how more could be done with light to add to its defeat of analysis paralysis (perhaps making the timer go faster on deeper dungeon levels), but that could have also sorts of unintended side effects that might be counterproductive:
What I will actually focus on is far more pedantic and way less impactful: The titles for Thieves.
The Lawful Progression is:
Footpad > Burglar > Rook > Underboss > Boss
The Chaotic is:
Thug > Cutthroat > Shadow > Assassin > Wraith
The Neutral is :
Robber > Outlaw > Rogue > Renegade > Bandit King/Queen
Being finnicky about criminal terms and theme I think it could be better redone as:
Lawful:
Burglar > Rogue > Highway(wo)man > Underboss > Boss
Chaotic:
Robber >Cutthroat > Shadow > Assassin > Wraith
Neutral:
Rook > Thug > Outlaw > Renegade Bandit King/Queen
Why?
Footpad is just another term for Robber, stealing with the threat of violence. This change makes the "Lawful" track the sort of people who could be considered anti-heroes in the right circumstances. Highwayman/Highwaywoman is a bit of cultural dependent one, much like outlaw (due to Robin Hood) but outlaw seems like by definition it couldn't be lawful.
Chaotic becomes those who employ murder and violence primarily.
Neutral becomes those who are self-interested without a primary focus on murder (unless you went with the historical origin of Thug from Thugee, but most people used the term to mean Goons, before THAT term also got changed) but who also aren't opposed to it.