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.
The earlier drafts were much more like that. They were largely a collaboration between a few web companies who had deployed OAuth 1.0 with Dick Hardt who had written WRAP at Microsoft. One of the major design goals was producing a protocol simple enough that client developers would not have to use libraries.
But the OAuth 2.0 spec we were working off of is now eighteen months old and as Eran said the vast majority of those contributors have drifted away from the effort over this past year :-\
I think the obvious next step was / is to make OAuth 1.0A into OAuth 1.1 by mandating TLS/SSL and declare the SSL-mimicking parts like the timestamp, nonce optional (i.e. ignored). Anyone can just do it by fiat, since it will be backwards compatible with OAuth 1.0A clients. They'll just send the proper timestamps and nonces, but you are ignore those fields.
I found those fields were 90% of the problem with OAuth 1.0A implementations. Maybe there's security value in those parts in an SSL environment I am missing, but I doubt it since SSL does the exact same thing.
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.