It occurred to me today that there is an important philosophical difference between the older web service protocols like XML-RPC and SOAP think of themselves and the way REST does.
SOAP and XML-RPC spend a lot of their spec on format of their messages. In particular, how application data is serialized into XML. How that message is moved from one host to another is less important. Both protocols assume that developers will use HTTP, but certainly other protocols can be used. In fact, Leostream at one point used XML-RPC messages across a pipe rather than TPC socket.
REST on the other hand focuses almost entirely on how messages are passed. That is, REST is tightly bound to HTTP. Application data can be serialized in a number of ways, none of which are particularly important to REST.