Having written client code for multiple OAuth2 implementations, I can tell you: it's a total clusterf$%k, and for exactly the reasons Eran outlines: the oauth spec is a giant ball of design-by-committee compromise and feels exactly like the disaster that is XML web services and it's technologies.

We would be better far off it a single company/dictator (like, shudder, facebook) came up with a simple, competently designed one page authentication mechanism, provided some libraries in the popular languages and we all just went with that.