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.
It sounds like the FM mobile site meets your requirements, especially with respect to inbox zero housekeeping (that I don't practice). I think that's a long way from claiming that the gmail mobile site doesn't work, as the other poster stated.
I think it's nice that there is more than one email service. I don't think they have to be totally ordered by some objective criterion.
My muscle memory just seems programmed to swipe back in the direction I came; and this seems much more intuitive. It's such an annoyance that I've now disabled the swipe functionality.
Does Fastmail implement this better? Or maybe I'm just strange...
I agree about Fastmail's mobile interface though. Almost anything else goes through Reader View (currently trying out https://addons.mozilla.org/en-US/firefox/addon/automatic-rea... which seems promising). Just wish the Reader View could remember my scroll position …
One of the biggest benefits to fastmail for me is the buttons are labelled with text on desktop.