For example one thing I like in FastMail is that in the message list you can finger swipe an item to the left for Delete and to the right for Archive. In Gmail doing that is not accompanied by a drag, so it isn't intuitive and you can only Archive, but not Delete. I like my Inbox to be clean and I prefer to delete junk, no reason to pollute my archive with nonsense. And when viewing a message, FastMail's UI also has arrows for jumping to the next message, which I like.
Another thing that bothers me in Gmail is the message details. In FastMail's UI you get more details.
And also, Gmail's mobile UI has virtually no preferences you can adjust and you have to switch to the desktop version for it, whereas FastMail's mobile UI has most settings in the desktop version.