Apart from what’s already been mentioned (bundles, general UI, speed etc), the killer feature for me was how Reminders were integrated into the email itself. When an email came in that I couldn’t quickly resolve and move on, I’d write the short ToDo to myself about what I specifically need to do.

Back in Gmail I go to my flagged/starred emails and I know I’m supposed to do something about the email, but many times I’ve forgotten what what specific action was, so I open up the email, read through it and realize that I for some reason or other won’t take the action right now. Having to ”rediscover” actions in emails is such a waste of time. With Inbox there would be clear ToDos written, so I could easily scan a list of emails and pick the right one to take action on.

Also very useful with emails containing many actions within, but only one or a few are left unresolved, or an email with useful links for future reading, so easy to specify what in the email I should focus on and thus ignore the rest.

RIP Inbox. Miss you dearly.