Timely, friendly, and professional. Slack focus on what really matters during the interview process - the candidate's experience.
Having interviewed at several SaaS companies in Dublin, Slack have constructed their remote interview process very well compared to the competition. Many companies fail at the basics (sending the wrong briefing for role-plays, confirming interviews the night before, etc.). When interviewing at Slack, you trust that you're in the hands of a professional at every stage.
One piece of constructive feedback - some of the documentation (presentation briefing for example) is just a basic word doc, it would look better if it had Slack's branding. But I'm nitpicking here!