Like Slack, it has per user pricing. This means it's difficult for large open source projects to use it if they have any interest in keeping a permanent log, or even keeping a message history that goes back more than a few days.

Feature-wise, the "Actions and Decisions" tools sound pretty interesting. Definitely a pain point with Slack. We've been using Slack for a lot of our planning for the Longhorn PHP conference (https://www.longhornphp.com), and it's honestly not been great. Conversations & decisions about particular topics get buried; if someone wasn't present for a conversation, it's likely they'll never see it. I know Slack isn't a project management tool - we've been using a mix of Google Docs and Trello for that. But even as a communications tool it feels fragile and incomplete.