(disclaimer: I work for FastMail)
Sure we have folders rather than tags, which means you can't add multiple of them to the same message. Probably the biggest lack is that you can't manage IMAP flags via the web interface. Otherwise, our search is now very powerful (since about March this year) and allows you to build filters that show messages from multiple folders in a single view.
The difficult parts are:
1) UI 2) limits. We have a hard limit of 128 "user flags" because it's in 4 x 32 bit fields in a fixed-width data format. Subtract a few for our internal tooling, and you probably only have 120 you can use for yourself.
Would 120 be enough?
One thing that many older clients did was had $Label1 => $Label5. That was often enough for people... so I suspect 120 is probably fine.
I have an Enhanced account that is linked to my domain, and an Ad Free account where important mail gets forwarded to. The latter is accessible from my phone, but the former isn't, so if anyone steals my phone they can only see the last few messages I exchanged, and I can just disable an alternative login (similar to revoking an API key) to lock my phone out permanently. I also set up my default personality in the Ad Free account to send all mail through my Enhanced account, so every mail I send from my phone is automatically saved in my Enhanced account and even has the correct DKIM signature.
One question, though, and I'm sure a lot of people are curious about it. You are an Australian subsidiary of a Norwegian company, but your servers are in the US. What happens when an American three-letter agency wants to see the contents of your servers?
We've had a number of US-based law enforcement bodies over
the year try to get hold of our data without going via the
appropriate Australian bodies, and it doesn't work out for
them. In the end, they have always ended up submitting a
request for cooperation via the Australian Federal Police,
as they are required to do, and we respond to that request
in line with Australian law.
Also, since we're talking feature requests: 1) contacts syncing via CardDav and 2) Calendar.
Contact syncing is vital though. It's really a killer at this point.
Recent revelations mean there may be a few more Australians willing to pay a premium to host mail in Australia.
Also, thanks for your work on Cyrus.
It would be better if you have a more desirable .com domain name. I will probably buy an Enhanced account to use my own domains after free trial.
The only problem I have with FastMail is that I've been receiving an increased amount of spam in my inbox.
That said, I like fastmail very much: keyboard navigation, search and overall speed are great!
I just wish it had an address book and calendar that I could sync.
Those are the things I miss the most from Gmail.