What we are selling is not the software product — the set of all the features, in their specific implementation — because there are just not many buyers for this software product.
However, if we are selling "a reduction in the cost of communication" or "zero effort knowledge management" or "making better decisions, faster" or "all your team communication, instantly searchable, available wherever you go" or "75% less email" or some other valuable result of adopting Slack, we will find many more buyers.
That's why what we're selling is organizational transformation.
I have some nit-picks about the practical realization of organizational transformation that Slack claims to provide, but this is one of the few corporate memos that I've read that clearly and succinctly explains what a startup business is about and why.