I like the idea, but does the syntax have to be that terse? It's quite expressive but less readable than code (or JSON/YAML), and I'm not sure why that's useful in this context.
At the moment, my stance is that the syntax needs to be as terse as possible so that it can comfortably be specified in a URL, or in a snippet in a unit test. In the current master, there's a variant syntax that lets you use newlines (or arbitrary whitespace) wherever the more compact syntax uses colons. In time, I could see an argument for also expanding the single-letter abbreviations in this mode to make a more verbose syntax. There's also no reason why we couldn't add a way of describing requests by manipulating an object, which then compiles down to the request language.
Anyway, this is all still evolving - if you have ideas for keeping the language compact but making it less like line-noise, drop me a line.