What amazes me in Fastmail is how fast their web UI both on desktop and mobile. Especially compared to Gmail but it's also more responsive than Slack or Trello.
In my opinion, the only thing they could do better is to provide an API for their file storage module, and I believe that is in the works.
You get what you pay for, and while Gmail is a good web client it is far from the best. Fastmail beats it hands down as far as I'm concerned.