• no support for private message (pubsubhubbub, anything atom-based)
• inability to move relationships when changing service
• no standard API for application interaction
by leaving each of these (and others) out of scope see: http://ostatus.org/sites/default/files/ostatus-1.0-draft-2-s... they have created an ecosystem unfriendly to developers (who have to approach each provider separately to work out auth schemes and APIs) and likely to lead to vendor lock-in (because relationships can't be transferred and basic features are implemented differently in each system).
First, welcome to the community of people working on this important problem. I highly recommend you join this group to stay informed about what's going on elsewhere:
It's an "issue" group, not dedicated to any one protocol, service, or software package. Please, make sure you're a part of it!
A couple of notes:
1. We're working on including private messages in PubSubHubbub 0.4 and thus into the next version of OStatus. Understood that it's a big deal.
2. You're right, there's no standard API. ActivityPub is an attempt at that; see here: http://www.w3.org/community/activitypub/
Thanks for considering. Let's make sure we interoperate!
* for messaging why would you not use xmpp and/or smtp?
As for things that have been tried before, we have ping backs -- that IMNHO never really worked. And there's Diaspora that have yet to come up with a stable protocol -- and have an implementation that is pretty badly broken.
It's also a good illustration of going the "full http" route: publishing becomes easy; interaction (server to server) becomes hard if you want to have any kind of security in place.