I think that's one of the key differences between REST and other kinds of RPC architectures. I've used SOAP and JSON-RPC, both of which (at least in many imple

I think that's one of the key differences between REST and other kinds of RPC architectures. I've used SOAP and JSON-RPC, both of which (at least in many implementations) send RPCs as HTTP POST requests and receive 200 responses with any error messages in the body. They're just tunneling over HTTP. It's not necessarily wrong, although I'm convinced that leveraging the HTTP verbs and error codes with REST is a fundamentally better design for the use cases I've seen.