Unfortunately, the "nobody owns" feature of EMail is something Slack lacks and probably always will. Email is a federated, decentralized protocol of distributed mail servers that anybody can set up. Any email server can participate with any other email server. That's something that Slack will never have (it does interface with XMPP and IRC, but the service itself is centralized.)

It is hard to overstate how critical this fact is to Email's future. If you look at most of the canonical Internet protocols (SMTP, HTTP, DNS, XMPP, IRC, even BitCoin) - they have this fundamental feature in common. Even though they're part of the application layer of the Internet, they've become fundamental protocols that other technologies rely on. They are part of the infrastructure of the Internet.

I say this is someone who just started using Slack and loves it. I still can't see it replacing email, though.