Things that happen on Slack:
- general discussion (can't avoid, but mainly it gets lost in the wash)
- asking if someone has seen bug X, or knows a workaround / solution
- people asking devs questions about the software's expected behavior
- devs discussing implementation details, code review clarifications
- interactions with representatives of Bitcoin businesses
- planning, discussion, feedback on projects done by BU (e.g. the Great Firewall of China XthinBlock tests)
I find this forum better for long-term discussions of certain topics. Difficult to retrieve those discussions from Slack right now, but Slack has the advantage that you get to chat live with people you work with on the project, which is nice.