It isn't what is wrong with Hangouts so much as it is a list of things we like about Slack: 1) Channels - each message has a home. Notifications can be set to the correct level. 2) APIs/Integrations - We hook it up to github, pivotal, new relic, AWS and some of our own custom hooks. Reminders, alerts, messages - whatever we dream up that needs to be done and it is open enough to do it. 3) History 4) Past an image right in it without saving it as a file. 5) Voice calls.

Right now Hangouts is only for meetings where we want to share a screen or video chat. Most of the time a voice call is enough.