Why Slack is so popular
One thing to bear in mind that people seem to be ignoring.... Slack is popular because they pulled an Apple. They took all the bits that worked, and avoided the things that drove people crazy. eg:
-Very simple to create a new team and add additional users to that team.
-Unlimited team size, even on the free tier. You only pay when you need the extra functionality. Hipchat didn't even offer a free tier until it was overwhelmingly obvious that Slack was kicking Atlassian's ass. By then, Slack had already reached a dominent position in the market.
-For the most part, it Just Works(tm). You can use it in a browser, on Desktop, on most mobile platforms, and they all work pretty well. The only reason I know bugs exist, is because there's occasionally an update that says, "BTW, we fixed this bug." I personally haven't run into any.
-Further to the above, things that you would expect to just work, like being able to handoff from one device to another with full access to history, work. Before Slack, there wasn't a single system that could do that properly that didn't also have some other kind of serious trade-off.
-They are constantly making improvements that benefit users, and they clearly communication those improvements to users so that users know what to expect.
-No advertisements on the free tier.
The single thing that I don't like is that your data is not under your control. There is no self-hosted version, so we had to make it a policy to not share sensitive info through Slack.