I know they're quite different technically. But practically, what does ActivityPub unlock that was not previously possible with RSS and basic web tech stack?

I think I have an idea of the answer. RSS may provide a way for users to "subscribe" to content from a feed, equivalent of following and putting it in a unified feed.

But it does not have a way for users to interact with the poster, like comments or likes. This may be possible with a basic web stack though, but either users will have to make accounts on every person's site, or the site has to accept no user auth. (but this could be resolved with a identity provider standard, like disqus does)

I suppose another thing activityPub does is distribute content to multiple servers. Not sure if this is really desirable though?

Anyways, did I miss anything?