I can federate message across hosts, fine, but I can't move my username to another host, so it doesn't work like email, because with email the namespace is defined at the DNS level, and I just forward requests to whatever email host I want to use.

Being able to leave one server and join another while maintaining an identity (say, a public key for instance) is on Matrix's to do list, they haven't decided how to do it yet afaik.