Having recently switched from a workplace that used Slack to one that uses Hipchat, my first answer is "everything". It's less reliable, and reliability is my number one desire in a communications tool. And there are just so many little UI and UX details that Slack gets right that it's a pleasure to use, whereas Hipchat is generally an irritation and a disappointment for me.
I guess one way to explain it is that Hipchat feels like an enterprise product, and Slack feels like a consumer product. The difference being that enterprise products get used because some high-level person says, "Lo, all my vassals shall now use Hipchat." Which means that user experience is secondary. Whereas consumer software has to earn each user, meaning that it works harder to please and support those users.
This is actually a great description of Trello v. JIRA, as well. Trello may not have the burndown charts, but what it does have is dead-simple list management, and it turns out that's what's most effective for managing tasks.
JIRA's BDUF approach to ticketing/bug tracking pleases middle managers whose job is to spend all day clicking around arcane interfaces and finding a way to generate a report that shows their team is highly productive, but it's painful for actual doers to get in there and move stuff around, which means it rarely gets done, which means that the tracking is not very reliable, which means that the value of the bug tracker is dubious. The most important feature any bug tracker can have is that it's low-friction enough that most people will actually use it.
JIRA has tried various things to make this less onerous, including GreenHopper/swimlanes, an attempt to remake JIRA into a Trello-like drag-and-drop interface, but it just never seems to click the same way. For example, today, when I tried to move a ticket in JIRA from the "New" swimlane to the "Done" swimlane, I got a "WORKFLOW EXCEPTION".
What's funnier, in an attempt to make things easier Atlassian is alienating power users. I liked Confluence's mix of html and their own markup well enough to write all my pages that way. Then an update hit and WYSISWYG was pushed down everyone's throats hard. To the point where you had to jump through many hoops to use even a limited subset of the markup that was previously available.
Product managers need to learn a simple lesson: if a user wants to use power user features then you should let them because there's a good chance they know what they are doing.
Same here. I'd written a Markdown-to-Confluence converter so that I could use a local toolchain to autogenerate project documentation. It worked brilliantly until I came in one Monday and found that the editing mode I'd used to upload my documents was gone. On purpose. For good. I invented some profanities that day.