I'm not OP, but the answer to your question is in his post: the main thing I miss about Gmail is the tagged labels, which make a lot more sense than folders and are quite an improvement in usability.

Other than that, I'm much happier with FastMail.

You'll be happy to hear that the JMAP work we're doing will make labels a lot easier to do on the FastMail interface

(on the downside, we can't yet account quota cheaply for that, so a message with multiple labels will count multiple times towards your quota usage at first)

That sounds intriguing! Is there a write-up about what you are going to do with labels?