I might have an answer, as someone who was unimpressed by Shadowdark for the last year, who wondered the whole time "What's the big deal when this is basically OSE, Labyrinth Lord, Castles & Crusades, Swords & Wizardry, etc, with a torch timer gimmick?" "Why did this one game matter when it's basically just a collection of rules that we already know, kinda don't need to even look at the book to run it because it's second nature to any GM who's run D&D from 1974-1999?"
And I think I've finally come to the answer I've been struggling with since Shadowdark's release: we don't need it ... it is nothing special. It's D&D stripped down to the most basic it can be. There's no matrices. There's not a list of class features you get every level. There's not a list of ability score/alignment/race restrictions to qualify for a class. There's not percentile skills for thieves or THAC0.
It's what I would have in my brain if I had to recreate D&D from memory. It is the fundamental core that is left when all b.s. is stripped away.
And let me assure you, I was a critic of this game. I was with Castles & Crusades since the White Box. I wrote for Swords & Wizardry. I have backed several OSE Kickstarters. All those have orders of magnitude of junk in them compared to Shadowdark - I just didn't realize it until I decided to write this post at this very moment.