A lot of questions are of the form "XML-RPC does X, but it isn't clear how to do X in REST", and the answer to most of those questions should be "You wouldn't design the system to do X when taking the REST approach."

Think of it this way: XML-RPC is structured programming, which is all about the subroutines you call, the data you pass to them, and the data you get back. REST is object oriented programming, which is all about the data and the methods you use to interact with the data.

In REST, a type of resource is like a class, a specific resource is like an object, and all objects have some/all of the same basic methods: OPTIONS, HEAD, GET, PUT, POST, DELETE. These methods always have the same purposes:

    OPTIONS: which methods does this resource support
    HEAD: what is this resources metadata
    GET: get a representation of the resource
    PUT: store a new representation of the resource
    POST: create a new resource and return its URI
    DELETE: get rid of the resource

The HTTP standard defines the responses to all of these methods, except for the specific representations. The defined responses include the status codes to return, the resource metadata (various HTTP headers), and how to specify the Content-Type header which identifies the representation being returned. The standard also specifies how the client should tell the service which representations it knows how to handle, in order to negotiate and find a useful content-type.

Where REST services differ are:

   1) What resources are available
   2) What are the representations for each resource
   3) What additional arguments are supported for each method and representation. (Query parameters for HEAD and GET, content body for POST and PUT.)

Another typical pattern for REST services is that, for every resource type, there is also a collection resource. To create a new resource, you PUT to a specific resource URI, or you POST to the container. Which you choose to implement as a service designer depends upon who's responsible for deciding the URI for the new resource; if the client decides use PUT, and if the service decides use POST.

One commenter asks (paraphrasing) "How do I use REST to call a function and pass an array as the third argument, and get back a sensible error if I pass a hash instead?". My response is that you probably wouldn't be doing something like that with a REST service. It's difficult to be more specific without a concrete example, but right from the top I'd say you're not calling functions in REST, and you need to invert your thinking in order to switch between XML-RPC mode and REST mode. (Similar to the way you can never really work well with OOP as long as you're thinking in terms of subroutines rather than classes and objects.)