When REST folk compare their approach with SOAP-based Web Services (which they often mistakenly call "SOA", not realising that REST is an equally valid way to do SOA), they often refer to a "uniform interface" that REST provides that SOAP does not.
The response of the SOAP crowd (when there is one), is "What's a uniform interface?" (They'd be surprised to hear that they have one too, which is the topic of this post.)
The uniform interface refers to a standard way to do something regardless of what the specific activity is.
As an example, if I asked what the SOAP interface would be like for a service, and I refuse to say what the actual business function is, the SOAP people would have to stop after describing the service at a very high level. There's a SOAP message with a header and a body, and what goes into them depends entirely on what I want to do. The general semantics of a SOAP message are "Process this". That's actually too general to be useful. Any further detail they could give me would depend on what I tell them about the actual operation I'm trying to perform.
In contrast, the REST guys can actually tell me much more about what my service is going to look like based only on the rough contours of my business function.
They can tell me that what I operate on is a resource that will look like this:
They can tell me that I will be using one of four verbs (GET, POST, PUT or DELETE).
They can tell me that my response status codes will be one of some 50-odd pre-defined status codes.
Plus they can give me other general tips that will apply, such as a GET operation will have no side-effects, a deferred processing request will return a "202 Accepted" status when successful, a POST request will be to a resource collection, and its response status code will be "201 Created" when successful, and the URI of the newly created resource will be found in a response header called "Location:", etc., etc.
That's actually quite a bit of information considering I haven't said anything yet about the business function I'm trying to implement.
That's what is called a "uniform interface". It's a structured framework within which application designers have to work. Far from being restrictive and limiting, these constraints actually make things easier for the designer because they provide standard patterns that can be reused in similar situations and deliver predictable results.
So far, so good. What the REST side doesn't understand is that SOAP-based Web Services offer a uniform interface too. Obviously not for the application itself, but for qualities of service.
Ask a SOAP service designer how they plan to implement reliable messaging, and they can show you very detailed WS-ReliableExchange headers that are virtually the same regardless of the business service. Ask them about security (encryption and signing/authentication) and they can show you the detailed WS-Security/SecureConversation/Trust headers required, and these too are standard and unchanging.
Security and Reliability are "Qualities of Services". SOAP-based Web Services have standardised these, and are also on track to standardising Transactions through WS-AtomicTransaction and WS-BusinessActivity (the latter deals with real-world transactions that require compensating actions to "roll back"). [Some may argue that these have in fact been standardised, but there isn't yet a WS-I Basic XXX Profile for these, like there is for Security and Reliability, which means there is no benchmark yet for interoperability in these areas.]
This isn't something to be sneezed at, because a uniform interface leads to improved interoperability, which in turn reduces the cost of integration. From my own experience in IT, I know that integration cost forms a very high proportion of the cost of most projects. Indeed, Project Tango proved the interoperability of Java and .NET-based Web Services using declarative statements in policy files to have these QoS headers automatically generated. Now that's cool.
REST promises to reduce integration cost and thereby overall cost, mainly though the mechanism of the "uniform interface". However, REST does not have a standard model for Security or Reliability, let alone Transactions or Process Coordination. SSL is a rigid security mechanism that is adequate for most, but not all situations. And while it's true that Reliability is probably better implemented through patterns like Idempotence than through the TCP-like model followed by WS-ReliableExchange, it requires an application designer to consciously build it. Transactions and Process Coordination are also missing from the REST lineup.
That's the challenge. Both SOAP and REST have standardised a part of the service interface, but not all of it. REST has standardised the interface of the application's core logic. SOAP has standardised the interface for Qualities of Service. What we need is a model that provides us with both.