I would expect a few simple things - GET doesn't change anything in the server's state (nothing externally perceivable anyway), a 4xx error code means I sent so

I would expect a few simple things - GET doesn't change anything in the server's state (nothing externally perceivable anyway), a 4xx error code means I sent something wrong, a 5xx means the server has a problem, a 2xx is some sort of success. That's about it really. Everything else is arbitrary. There'll likely be some kind of attempt to structure URIs so that resources follow some sort of scheme, but since most APIs are really RPC instead of information access this is always a nightmare.