Every* developer has wanted to create a product like Slack. A product that is not revolutionarily different but a better designed version of existing products (Zapier, HipChat, Campfire). That is why there are an abundance of Twitter, RSS, Podcast, you-name-it clients. Everyone has their own take on how these utilities should function, and they think that their version/approach is the best. Their version is good enough to make it and build an entire business upon a single utility app.
In addition, Slack has achieved subscription-based pricing tiered on the number of users and features you require. This pricing strategy is one of the best formulas for a sustainable software business.
Over the past few years, I think we’ve seen an abundance of apps try to do what Slack has done and fail. Well done Slack!
*Well of course, everyone is a generalization, but I think you get the point.