How is that better than email? If you want people to check it at their own pace (async), then use an async method. Slack expects everyone to check it constantly. At least in my experience :(
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.
Not to mention the integrated file attachment (inline screenshots/video, anyone?) and impromptu /call feature to trigger a voice call or screen share.. there isn't even a reasonable competition there anymore.
If you feel overt pressure from expectation to read/respond immediately, it seems you may need to establish a Slack-usage culture that works for your team (or express that the current expectation level doesn't work for you and is affecting you negatively). For me personally, I don't necessarily expect anyone to respond immediately, even if I "Direct Message" them and they are Online. While I respond to most stuff promptly, it's never a problem if I don't.
> 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.
