> It insists on having a chain of updates from a single user, which feels unnecessary to me and something that adds bloat and rigidity to the thing — each server/user needs to store all the chain of posts to be sure the new one is valid. Why? (Maybe they have a good reason);

I assume this is so a relay can't manipulate your messaging by picking and choosing which messages to forward; they'd have to forward messages [0-N].

Edit:

> sig: <64-bytes signature of the sha256 hash of the serialized event data, which is the same as the "id" field>

Signed hash rather than a mac - might be vulnerable to an extension attack