The interface on Inbox is _much_ less cluttered. I spent an hour turning off things in gmail trying to clean it up after the announcement, and it still looks too busy.
Other specific features are missing that help with email fatigue. As far as I know, there's no way for me to tell gmail to group a set of emails together and only alert / show them to me once per day at a certain time, like you can with Inbox. I can't create custom bundles to hide large groups of emails I don't want to accidentally bury other content (without actually removing them from the inbox). Gmail kind of has the "tabbed inbox", but you can't configure the categories with the flexibility that bundles had. In Inbox, I feel like I can control how much email interrupts me without also missing anything important. I don't get this same feeling with gmail.
The Inbox product as a whole had a strong focus on trying to reduce the interruption of your email and keep your inbox clean. While gmail has taken in some of the features, it hasn't absorbed the core goal of the project.
I think it’s the UI mostly. It’s nice having all your messages categorized in bundles, grouped chronologically, then sorted by priority. With Inbox I find that I skim through way less crap to find emails that need attention.
