I think what Slack got right was the on-boarding/time to wow. In a sea of vanilla, sterile enterprise products was this tool that just seemed to be having fun with it. For small companies, particularly in the SF area, it felt like it understood their vibe. For larger companies, it felt like they were joining the cool kids. My team uses Slack now and honestly, it's nothing _that_ special. It's a chat room. It's done well and has nice integrations (that's their product moat), but adoption is so key and they nailed that.