REST and reliability
With this simplicity, REST also leverages the ubiquitous HTTP protocol as the underlying mechanism. More and more people seem to like this, including me.
However, the big question for me is: how do you make this reliable? Imagine that you integrate 4 systems in a REST style. You would be using HTTP and a synchronous invocation mechanism for each service. Now comes the question: how reliable is this? The answer: less than the least reliable system that you are using! More precisely, availability goes down quickly because your aggregated service fails as soon as one of the services fails...
With transports like JMS you can improve reliability, but how do you do REST of JMS, given its close relationship with HTTP and URLs? That is the problem with REST for me.