Have some kind of real-time interface. The chat-like style Spectrum.chat used was great for this (apart from Spectrum (source linked) being slow as hell).
Provide plenty of single sign-on. IndieAuth on by default could be a big help.
Have a cheap/free (at least sponsor open-source projects to be free), dead-simple way to host your own
Other nice-to-haves:
Fediverse support of some kind.
Integration with Discord (hell, using Discord's forums, and bi-directional! It's possible with Discord's API, last I checked).
