Current problems I, personally, have with OStatus:

1] No reliable way to poll external servers for capabilities
2] Transmission failures are not caught and there is no mechanism for retransmits
3] A considerable amount of meta-data is leaked.
4] Extensibility degrades poorly.
5] Existing implementations all disagree on the "standard"
6] A lot of stuff can be put in the fields which really should not be
7] Much larger attack surface than is neccesary for the protocol