Earlier quoted context omitted.
Inbox remains for me the best product they killed. It was in every way superior to Gmail and offered something I haven’t seen since.
What was it that made it so good? I had already degooglified my online life when it got released, so I never tried it out.
For me the killer feature was the way emails were (automatically) contextually organized. Instead of your inbox being chronological, it would create "bundles" of emails that were related based on sender, or subject line, or content. You'd have a bundle for each of: your JIRA emails, github emails, bank emails, cron messages, threads with your boss or coworkers, travel emails, receipts, etc. Bundles were created automatically and accurately. Labeling like in gmail would let you create custom bundles if needed. You could archive/label/refile/mark-read an entire bundle of emails at once. It was extremely efficient.
IMO there's too much context switching when your email is sorted chronologically. Much easier to deal with all emails of a certain type at once. You can (manually) organize your email with labels or folders, and go through each one, but it's just not the same.
EDIT: there were other neat features like pinning and snoozing that have since made their way into gmail and other apps, but IMO they worked better with the bundle system than they do without