> Being able to bring someone into a discussion which they can quickly respond to (or browse back on later) is way smoother a process than having to send another "adding Bill to the thread" Reply-All every time you want to add someone.

That could be solved by using NNTP instead of email. Plus, with clients that handle proper message threading and search, you can get something far better than what Slack offers in terms of going through a discussion that has already taken place after the fact.