There is not a lot to see here (yet). Some images or a demo video would help understand what exactly is going on. I think a nice short gif of the app in action on the landing page would go a long way to immediately tell people what your product is.
Email is async already, and as long as you don't enable any notifications, you check emails at your preferred time. Same for Slack in principle, except maybe some coworkers expect quicker responses, but that depends on company culture (many are clear about Slack still being for async communication without immediate replies).
Email is not threaded! I have to always group emails to catch up on the conversation. Email is recent first and in order to read the older messages you have to read bottom up to catchup.
Tools are like a flowing river, they have a natural flow. Going against that flow is like swimming against a tide. Slack’s natural flow is urgency which is bad for focused work. Hence the need for threadli, which is essentially opensource Basecamp (Messages) or Twist really.
Maybe you got used to bad email clients :) email has always been threaded, as far as I remember... Then Outlook broke email and Gmail finally killed it. Now everyone tries to reinvent mutt, pine and Thunderbird :)
