Given that 'Comments' on HN are different than comments on Facebook, comments on a sales invoice, comments in a threaded forum, and comments on a pull request, how will this get standardized? On their face they may have relatively similar goals, but they have different semantics, rules and ways of being displayed.
On top of that, what happens when I create a new application, with a new object concept that's not standardized? Do I have to wait for it to get standardized and implemented in this magic client before I can launch? Or do I have to build a custom client anyway, entirely negating the point of using standardized stuff in the first place?
It's conceivable that with enough effort, some client could understand the semantics of all object types that are contained in a standard, and the standard is big enough to cover ever situation ever possible.. But then what? Who wants to build an app that basically looks and functions identically to your competitors (who are all using the same semantic objects)?
I don't have an example of a public API that can be used by a generic client because no one is making them. I've played around with them in a few projects and there are rough edges, but no reason we can't move forward to them. Google already use 'real' REST for some of their mobile apps as far as I can remember.