Infrastructure Debt - Slack's one of those places that both amazes and astounds. I'm sometimes surprised by the lack of infrastructure inside the company. The leadership team are aware of it, and the systems are in the process of being built... it just sometimes doesn't keep up with the speed of the growth.
Product-centric - It's a fairly typical company whose origins are centered in product. We're in the phase of moving beyond feature release/adoption, and need to understand true customer value. This is something that most product companies will go through as they grow, and Slack is no exception. Sometimes the product won't "sell itself" as some people believe.