- User experience. In 2016, I'm not using the hacker's AIM to collaborate. I know many people love it, but there are seriously better options. Syntax highlighting inline and file attachments that Just Work are fantastic.
- Product development. I want products I use to be continually improving. Not yearly but weekly. If something breaks, I don't want to fix it because I'm not here to build a chat program or manage one.
Things I don't care about:
- Closed Source. A dedicated team paid to work on the product full time, financially incentivized to expand the ecosystem is a major plus.
- A walled garden. This sounds like the same as #1. Slack allows many integration points, including chatbots that can be programmed to do anything. I'm not sure it is that closed off.
There is a reason large teams collaborate over Slack as opposed to IRC. If IRC filled people's needs, they would use it. It doesn't.