This isn't about open source vs. proprietary. Our challenge is getting engagement from the members to build culture and levels of activity. In a 24 hour period using slack and reddit I've marshalled more resources and commitments of peoples time towards the Pirate Party cause than I have over every time ive tried to engage on our IRC, which is in a state of serious cultural tumbleweed and way too serious to encourage a willingness to fail and explore.
We need memes, shitposting, laughs, fights, and cool. Slack does this well.
The dialup access... seriously? Are we setting our bar there? Because it comes at a cost. Not technical cost. Organisational culture.
Anonymity - it requires an email address. I dont think thats really a breach of anonymity, disposable email addresses are not a new thing.
Platform: Mac, Windows, iPhone, Android and Web Browser... thats pretty good coverage. No it will not work with a non gui Unix implementation like IRC will. But again, is that our bar for a communications tool?