Gmail has "tabs" or whatever they are called which automatically aggregate your emails into different panes. The automatic part didn't work well for me. I'd often find important emails in my "Social" tab, which led to me checking all tabs to make sure the machine learning didn't incorrectly categorize an email.

Hey gives me control over that, and it has been fantastic for the 11 days that I've had an account.

There are other features I love, but the feed has made the biggest impact so far.