There are a few reasons why I'd never switch back to gmail:
1) Reliability: Gmail sometimes had issues with Apple Mail where messages would show up twice, etc. Since switching to Fastmail I had no more problems.
2) Customer support. I once had an issue where all my mail disappeared from my inbox in Gmail. There was noone I could contact, all I found was a user forum with other people complaining about the problem. I never got my email back from the archive to the inbox. When you have a problem with Fastmail, or can't find what you are looking for in the docs, just contact customer support.
3) Backup: Fastmail allows you to restore email from backup (for a limited time). Really nice when you accidentally delete an email.
4) Web interface. I assume that's a matter of taste, but I really prefer Fastmail.
5) Folders instead of labels. I don't like Gmail's labels, and I'm glad Fastmail has proper folders.