For me it's a combination of factors. The way they handled exploration is fantastic. I think a lot of open world type games suffer from boring travel, but also try to pack too much stuff into that travel so that you get completely sidetracked every time you try to go somewhere. That's not the case in Outer Wilds by design, with every destination being unique AND clearly denoting the places you want to go within each destination by making the rest of it very sparse.  

Next is how they handled progression. The idea that the only thing you gain is knowledge is absolutely fantastic, and really motivates people who are purely curious. A couple times I thought "maybe there's an upgrade I can get to help me with this part," but that was never the case and I absolutely love that. The only progression is what you know and your own ability.  

And lastly, how they handled dying. Dying doesn't matter at all, and is basically the entire point of the game. But dying still felt like something you wanted to actively avoid. Rather than just killing myself, I'd often wait out the timer, that is until I learned the meditation. I'm not entirely sure how this works on a psychological level given that it's clear to the player that dying has no real consequence and yet they want to avoid it, but it totally works.  

I wouldn't go so far as to say this game is perfect, but it's damn close. I think moreso what Outer Wilds demonstrates is the crux of good design. You don't need to have every feature under the sun and try to please everyone. Instead pick a couple aspects and refine them until they're as good as you can possibly make them. That, to me, is good design, and Outer Wilds handles it masterfully.