I reread the article and I understand that the criticism is quite weak; for me the most important part here is not that Slack is closed source; but that Slack is, IMO, just another member of the dot-com bubble 2.0, and, as with many other tech unicorns, the investors behind it are trying to convince us that we must use it just to justify the valuation before the IPO.
in the near future we'll see if my concerns are justified or not; anyway, good-old IRC will still be there after the new tech bubble bursts.
Slack is more than just chat; AFAIK, the idea behind it is something similar to Google Wave, but done in a better way.
to be honest, I'm really tired of following up many of these new ideas; I'm old and I prefer to deal with boring (and proved) technologies (like email, forums and plaint-text chat) as each one of them solves a specific problem in a very good way.
I really don't understand why somebody wants to be always on for work; communication using smartphone apps is most of the time superficial as you don't have time for reflection; but that's me and I'm a "weird" guy in present times.
I think there's no silver bullet for solving communication issues with newbies that don't want to learn Plone, but to solve some immediate problem for their current employer, some sort of Stack Overflow syndrome.
if somebody needs/wants to use Slack, just use it; but let's not forget the main issue behind this long thread: should we switch away from IRC? why?
I agree, as long as we don't sell ourselves in the process.