The protocols that exist in NSQ now are designed to be the simplest implementation that worked.
You've correctly pointed out some of the issues. At this stage, the distinction of producer vs consumer was mostly so that you could publish at all without having to use the HTTP interface. For our use cases, in particular taking advantage of the /mput endpoint, we aren't even using the TCP based publishing protocol.
Re: your point on sending metadata with SUB commands. I agree its a bit ugly. We actually intend on improving that aspect by instead sending the data in the form of an IDENTIFY type command upon initial connection. That information is used in the various administrative UIs and endpoints.
I'm going to do a bit more reading on MQTT, thanks for the link.
DISCLAIMER: I wrote the RabbitMQ in Action book.
We wanted a brokerless, decentralized, and distributed system. NSQ has no broker or SPOF so it is able to route around failures.
That said, I think RabbitMQ is a good tool depending on your requirements. I can imagine a broker proving useful in situations where you may want strict ordering or no-duplication. Those were tradeoffs we were willing to make for fault tolerance and availability.
Also, given the fact that we were already operating infrastructure built on simplequeue (which is also distributed and brokerless) we found it more appealing to evolve rather than replace.
There is a bit of an impedance mismatch since now you have to go out-of-band to fetch information. Not saying it's a bad thing; that's essentially also the role of DNS and LDAP, among others, so it is a fairly common pattern.
You bring up an important point though... What is important is the topology we're promoting and the platform we've built to support it. The actual queue is less important and it made sense for us to own that piece to achieve our goals.
The wire protocol seems more complicated -- why distinct protocols for producers and consumers? Linebreak-delimited headers are error-prone and ought to be banished. Why transmit the hostname with sub requests -- is another system requesting jobs on behalf of workers?
Then the topic/channel distinction seems artificial when wildcards would suffice (and provide much more flexibility), eg topic/*/channels or topic/channels/# in the MQTT parlance. MQTT also has more fine-grained delivery guarantees via its QoS levels. All this with a header structure that's as small as two bytes.
edit: While MQTT is pubsub, as long as the system is under your control, you're free to change the semantics at the broker side from "broadcast messages to all consumers" to "rotate messages among consumers."