I've been using Slack for almost two years and it's fantastic.

I've been invited to HipChat at a different company and I gotta tell ya it feels like a stone age. Off the top of my head, here's a list of issues that make it a PITA to use HipChat:

1. Integrations can be installed to a channel only by a channel creator. (Why?)
2. What's more, after I have created a different channel, there's no way to pass admin rights to it to a different person.
3. Push notifications don't work on my Android phone. I get emails every time I'm mentioned. Not just me - my coworker too.
4. Notification sound is annoying and cannot change it to something else.
5. "New chat" button in HipChat sucks. Slack's interface is terrific. Thinking of a private conversation? Your eyes go to "PEOPLE" list on the left. Person you want to talk not there? Oh, you gotta look for "New chat" button somewhere else in the interface. Sam with rooms. "New chat" button sucks.
6. Video call quality is low. When there's a transmission problem, both video and audio are affected. Google Hangouts seem to use almost the same technology, yet Hangouts sacrifice video first, not audio. 
7. HipCha dynamically controls the audio recording volume. I can see that in pavucontrol. This makes the noise very loud when a person doesn't talk. We stopped using HipChat video calls after a couple days, it's too bad because of the previous two things.

And some minor things:

1. No way to disable emoticons.
2. Upload doesn't start until I choose the filename.
3. No minimize to tray (at least for Linux client)
4. Having to use /code instead of well-known Markdown's ```. No bold or italics. And such.

Hopefully this is helpful for y'all.