One of the problems people have with the REST architectural style is its lack of definition of a common data format, but that is actually one of the powerful features of REST based systems.
It's very attractive to programmers to have a single all in one protocol and encoding format, programmers like to think in terms of functions and parameters which is what you get with XML-RPC and SOAP. They have their own way of representing common data formats in XML, meaning you can pass values back and forth without having to write or decode any XML, the libraries do it all for you.
The problem comes when you want to pass something more than a simple data value back and forth. If you define a data format like XML-RPC or SOAP do, then you'd better make sure it covers all posibilities. How for example do I pass a binary file back from an XML-RPC request? I have to encode somehow into the XML-RPC XML response. Fun.
REST doesn't work that way, which is alien to most programmers. It works with URLs and representations, so rather than calling a function with a set of parameters, you're fetching the state of a resource. Working with REST requires you to write applications in a RESTful way, rather than just bolting HTTP based RPC onto a procedural way of thinking.
With a REST based system, my HTTP response body for that request is just the image itself, no messing around with XML, HTTP handles it all, and why not, it's proved technology, millions of images are already transfered that way everyday.
XML is great, it gives us a way of formatting and encoding arbitory data in a way we can easily get it back with standard tools. But XML is not the answer to all of our problems, it's a tool suitable for some problems, you wouldn't use a hammer to fit your broken window, don't use XML to do things other formats do better. That said, XML is great, the best thing about it is the X, the extensible, the fact it lets us define our own formats for our data, with SOAP and XML-RPC we throw away that power.
One of peoples favourite arguments about having a standard data encoding is that it means it saves you from writing extra code. Brent Simmons said recently:
[REST] doesn't say anything about how data is formatted. So you have to parse documents. With XML-RPC, assuming you have a library, you never have to parse anything.
Well, assuming you have an XML library, you never have to parse anything, that's the reason for XML, to create a standard document format you can use off the shelf libraries to parse. It doesn't matter what tags are in the XML, a decent XML library will get you the values and structures from the XML document in a nice native format for your programming language without you having to lift a finger.
Further more, REST doesn't stop you from using a standard XML data format if that's what's best for your application, although it is most likely you'll end up with a meaningless response package that only makes sense in the context of your application and is less RESTful than it could be. Instead of XML containing users with addresses, you'll have arrays full of strings, not very semantic, not very RESTful, not really what everyone had in mind when XML was invented.
So get over the common data format idea, embrace custom response formats that make sense with respect to the resource being sent, and think RESTfully when designing your applications from the ground up.