I'm not aware of any particular feature that doesn't fit into XMPP with a couple new namespaces at most.

And before you get too down on XMPP, bear in mind that anything that standardizes half-a-dozen chat clients will end up pretty nasty on its own, too. It all looks simple when you're a user of the client, but under the hood chat systems get... "interesting". In the apocryphal Chinese curse sense.

The best thing since IRC is IRC.

Slack has the advantage of maintaining message history server-side instead of client-side, so you can catch up on things you missed without running a client in a screen session (or a bouncer, or a logger, or...). This functionality is part of the core protocol.

Is there a similar standard for IRC? If not, then IRC is serving a different use case.

It's trivial to log IRC, but it is hard to integrate server-side log display and search with prebaked clients.