Just to add on to the email discussion: right now the one issue with Slack is it's hard to be discussing multiple things concurrently. Conversations overlap and there's no "chain."
Luckily, though Slack has been teasing for months that they are working very hard to make a message-threading system. I guess the reason they are taking so long is that they're really trying to get it right.
http://www.theverge.com/2016/4/13/11...at-app-feature