One point in my recent REST vs. SOAP — there are more nouns than verbs! post I don’t seem to have communicated well. Let me try to augment what I said…
Imagine two software systems that both process the same amount of information and generally do the same thing. The first of those systems is not a web-based system, and thus does not allow anybody to address any part of its functionality or information using URLs. The second of those systems is exposing absolutely everything as URLs. Which of those two systems is more conducive to third-party innovation?
Well, think innovation a la del.icio.us or Technorati Tags. It’s very obvious that the non-web-based system cannot work with those innovative services — there’s nothing that those services could refer to, and this kind of innovation would not occur.
I meant to conclude that if two otherwise equivalent information systems expose a substantially different number of "things" as URLs, all other things being equal, the system with the more URLs would provides more points to which innovation can latch itself, del.icio.us being the prime but by no means the only example. (One could argue that a SOAP-based architecture might allow the same number of latching-on-points but they are much more "hidden" behind so many messages instead of being "up front" as in case of REST. The conclusion is the same).
People generally seemed to agree with my view that SOAP-based architectures expose fewer URLs than REST-based ones, and so I conclude that (outside) innovation can attach itself more easily to a REST-based architecture than a SOAP-based one. On the first glance, fewer URLs could mean that an architecture was simpler, but I’m assuming the exact same functionality for both systems for my thought experiment: one could say that the REST-based architecture simply has a much "broader" API… at least the easily and uniformly accessible parts …