I wonder what the drawbacks of standardizing a GET body would have been. CoAP already has it (which creates friction in building CoAP<->HTTP proxies).

All in all, I dislike the overall focus on the HTTP method when designing "RESTful" interfaces. If all we're building is, effectively, an RPC, why would the cacheability meta-information be the first thing we specify?

A absolute swats of middle boxes that will not get addressed ever. As industry, it's preferable to create something that is a hard break and makes players upgrade and give people a feature to argue for said upgrade