You send a request to a web indexer with a page you want added to the network, it scans your page for outgoing links and sends an email to the owners of any domains who are also on the service.

One can imagine making it a little more featureful, for instance indexing OPML blogrolls which would allow you to see whom a person follows and https://microformats.org/wiki/h-entry which would allow outgoing links to be categorised (like, reply, etc) so notifications filtered. With that, it would be possible to add a front-end site imitating one of the popular social media paradigms (Reddit-style or Twitter-style being the most obvious).

If there's one genuine design decision I would give for it beyond what is basically a cobbling together of existing interfaces, it would be to charge users per page uploaded. Likely very detrimental to growing the service, but I think one of the simplest ways to weed out spam and junk. A real problem with many web services is that the receiver of the message pays for it in terms of attention, where in other mediums the sender has to pay. Given that uploading crap is basically free, that's all you get. Increasing the cost of upload would weed out those endless AI summaries and lists of affiliate links.

The issue with ATProto is that it operates on too many layers. You can see lacking in what I have described here the concept of durable authorship, but this is a property of content not how the content is distributed. Perhaps someone will invent a standard way to sign HTML documents, in which case you could base user accounts on that instead of DNS. AT enforces this centrally but it does not need to. It is walling itself off from the common and decentralised software ecosystem of the web for no good reason.