As @Delapouite said on Github:
The added value over IRC is that you can post long messages, with code highlighting, links, tags…
Mailing lists are always a mess in comparison.
I will add other benefits include images, formatting, mobile interface, web interface, mailing list mode, rss feeds, time-shifted (not real time like IRC), embedding and history.
Another set of nice features is being able to reorganize posts (I recently reorganized @eraserhd’s awesome golf thread into two threads then deleted the original), and polls as seen on the Future Assistants post.
I can tell that there’s merely any activity on reddit, and this isn’t good. Often a good reddit community means that community actually exists and it is mature community. Most of new users of Vim are going either to StackOverflow or to Reddit with their questions. If newcomers will see that reddit is actually isn’t active place, and there’s some weird ‘discuss’ thing, that isn’t as known as reddit or stack, I suppose many would leave. That’s only my point, maybe I’m wrong. However I found that great reddit community often really helps in advertising since there are crossposts, and searching tools are respecting reddit as a mature service. I think that reddit may be a better place for discussions since it is nearly identical in terms of creating topics, except that there are no support for pictures maybe.
Mailing list mode is a rather critical feature for many users, myself included. (This reply is via email)
It was actually the core reason for picking discourse.