I often think that, via REST APIs and content negotiations, API can become browseable and self-descriptive but I've yet to see any example of such a thing. Furthermore, just because an API is restful doesn't mean it's well-designed, and doesn't mean it has descriptive APIs (the artist's track could just as well be in rdio/api/64D89CAC?q=42, that wouldn't inherently make the API non-restful)