IMO its origins in the gaming community, and all the moderation features that grew out of it, make Discord a much better fit for open source communities than Slack. For example:
- Individuals can block and report other users
- There are tiered mod levels
- Per-community pseudonyms, but a single account makes it easier to track bad actors
Markdown support, including syntax highlighting, is actually better in Discord than Slack already, too.
If you haven't checked out an OSS community on Discord yet here are a few:
I run three Discord servers:
- A tiny one for my company, which we use much like one would use Slack within a company, including voice and video chats.
- A medium-size one for the open source community around the company. It includes project-specific channels (three-way mirrored between Gitter and IRC thanks to the wonderful Matterbridge: https://github.com/42wim/matterbridge/), general channels, voice channels etc.
- A large (20k users) one for our company's (gaming-centric) userbase.
Discord is a fantastic tool that adapts to all three situations very well, scales really well from 4 people to 100k people. Its DM/friendslist system scales a lot less well, but is still very usable with 100+ DM channels. I have even created a personal (private) Discord server where I'm keeping a journal of what I work on, inspired by a HN post the other day (https://news.ycombinator.com/item?id=15823599).
Discord is scalable messaging UX done right. I'm a huge believer in what they do. (Yeah, if only it were open source etc, I get it; different problem, different story)
I much prefer it over Gitter for open source chat (Gitter's only real advantage is how well it integrates with Github). And IRC is... well, not in a good state today. IRCCloud.com does wondeful work but they're small and it's just not enough.
I just wish Discord would get phonecall support, but that part is probably not going to happen. It's doable with a bot though. PhoneCord (https://www.reddit.com/r/discordapp/comments/6hlesz/anyone_e...) used to do it, they were shut down because of the obvious abuse implications but I'd really like to hook up Twilio with Discord in a bot for my company, internally, so we can do phone conferencing from it.