Why is RESTful design thought to be hard? I said this during Sanjiva's talk at QCon, but here's my one line summary
RESTful design is like relational data normalization.
Even though both are driven by principles, both are an art, not a science. And the popular alternatives, unfortunately, tend to be driven by craft and expediency.
The analogy could be taken further: "good RESTful designs" today, of the WADL variety, are very similar to 1NF. With ROA and the "connectedness principle", we're just starting to move into 2NF territory, I think.
Witty aporisms abound: "The Key, the Whole Key, and Nothing but the Key, So Help me Codd" sounds a lot like "Cool URIs Don't Change".
We haven't quite yet found the RESTful 3rd Normal Form "Sweet Spot".
"Everyone knows that no one goes beyond 3NF", so perhaps RDF and the Semantic Web are REST's 6th Normal Form, because they "scare people". Amusingly, Chris Date actually seems to think so.
I just *really* hope we don't have to go through 20+ years of defending REST the way Codd & Date had to defend the relational model against unprincipled alternatives, a debate that continues to some degree almost 40 years after Codd's original paper. If, in 2037, we're still debating the merits of Roy's thesis, I'd rather be a bartender...