Personally I care about the security of the server (is it written in a language that historically has a good or poor track record), and I care a great deal about the client.

The Slack client regularly pauses for me while I am typing, randomly refreshes, and generally feels sluggish. Those aren't things I associate with Go. That said, I think this particular implementation's client is also written in Javascript, so I don't know that it will have a particular advantage there. But an open source implementation at least won't block alternative clients (e.g. https://www.kickstarter.com/projects/2137936555/taut-the-fas...), in a way that the Slack Terms of Service would seem to do.