The obvious answer is anyone can run email. Only twitter can run twitter.

If the join.app.net team gets some inertia - that might change, but right now - twitter is a single player (and point of failure) game.

I can easily change email providers and remain in contact with the same people using the service and address they themselves choose. If I grow dissatisfied with these services, I can even go to the trouble of hosting my own email server.

I do not see a competitor to Twitter that is interoperable with their service. Nor is there a distributed option. Gmail and Hotmail are platforms that grew on top of an inherently distributed service. Twitter is a data silo.

And another: I can easily download my contacts and email history and take them with me. It's portable.

That's true. It doesn't have to be Twitter. join.app.net would be a better alternative if it could pick up some momentum.

How is gmail or hotmail the email protocol?

That's your FUBAR right there.

Also, what's so complicated about (a decent) email (client)?

And what's so great about being a "[insert company here] bitch" [1] ? An open protocol is something to build on. Twitter isn't.

[1] http://techcrunch.com/2011/05/23/fred-wilson-be-your-own-bit...