I'm one of the Substack founders. My (obviously biased) view is that we are trying to do something that is genuinely important, and that we're willing to sweat the details in service of the people who use Substack. I'll try to give that quick barbell of lofty vision and practical details.
Big picture: we believe that what you read matters, and that therefore great writing (and great intellectual culture in general) is valuable. In our view the great promise of the internet is that it can unlock a flourishing of culture, but the first phase was dominated by a land-grab for attention which gave us the current landscape of ad-driven platforms which optimize for the wrong things and don't serve great writing. Substack is an attempt to create an alternate universe on the internet, with different laws of physics, that fulfills the original promise. Where writers can make real money by earning and keeping the trust of an audience who deeply values their work. Where readers can take back their mind, and decide for themselves who to trust and how to spend their attention.
Practical terms: we try to focus really hard on serving the people that use Substack. The writers, obviously, by giving them something that genuinely works. I think of this as "we do everything for you except the hard part". Which means if you can write something great, the job of the product is to handle the rest. This means not just the right features, but smart defaults everywhere that help you succeed. For readers, we try to make the experience very clean and frictionless, and communicate right from the get-go that this is a place that respects you and your attention, to the point that it might be worth paying for. The things others have noted - smart defaults, fast loading, clean design, etc. etc. are expressions of this. We're focused serving people, and we're not shy about using 'boring' technology (like email, for example) to make it happen.
Putting this together, the magic of the Substack model is not that it gives writers a way to get paid for doing the thing they might have done elsewhere. It creates a system where the kind of thing you do to succeed is qualitatively different and better than what succeeds elsewhere. For writers, that can mean getting paid -- sometimes very handsomely -- to do the work you actually believe in. It can unlock this for people who weren't professional writers before. And for readers, it means a lot of the best writing to be found anywhere is on Substack.
That's the theory at least! We're hiring, by the way: https://substack.com/jobs