I often think of SMTP as well -- robust message queueing and routing with built-in failover, backoff algorithms, and myriad tools to view all the parts involved. An RFC822 message's envelope can contain infinite metadata and its body can contain text, json, xml, base64-encoded binary, you name it. You can encrypt messages with PGP, or secure the transport layer if you can determine who talks to you. There are more implementations that one can count, it works on every platform, and it doesn't scare away the systems guys.
I've also thought of SMTP as the ultimate queuing system. It really covers all of the bases for persistence, async delivery, bridges to every kind of network imaginable (think back to FidoNET, UUCP -- revive the bang path!), built in fault tolerance, etc.
The only thing really missing is delivery order guarantee. It isn't uncommon at all for a message to fail delivery, get stuffed into a retry bucket, and then a bunch of other messages get delivered without problem before the retry occurs. But even this is just an implementation issue. The underlying architecture is nearly perfect.
