Sending mail without having to see your inbox
For the longest time, the way I handled e-mail was with the "mail" program which came with the system. It was simple, fast (it never did "hourglass" or "beachball" garbage on me), and easy to use. It also stayed out of my way and never tried to steal the focus. It also rarely crashed.
I switched from that to Thunderbird when running directly on my mail server was no longer an option due to a proliferation of IMAP-only accounts in my life. A couple of years later, I wound up on Mail.app after deciding to switch my primary "amusement" roles to a Mac laptop.
There's one big thing which is missing today. It's something that you just can't do with any of the graphical mail clients or web mail interfaces that I have seen. It is the ability to just do this:
$ mail firstname.lastname@example.org Subject: VHF instance Hey Someone, I was thinking about standing up another instance to capture VHF signals too. Do you need anything in particular or should I go with defaults? Let me know before Monday. . $
See that? I just dashed off a mail without ever seeing the state of my inbox. Try doing that in a mail client now. Let's say you've actually shut it down because the little unread-number-in-a-red-dot thing annoys you when you're trying to focus on stuff. Now you have to start it up. It's going to sit there and thrash for a while as it reads indexes back in from disk and starts hitting all of your IMAP/whatever servers.
Eventually, it will decide to listen to your request to open a new window, and will stop lagging long enough to take your input on the To: field without blocking on disk I/O to the address book. Then you can type in your message and send it off.
Now comes the hard part: you have to shut it down again without allowing any of the things it displays to get into your brain and screw up your "state". There's a lot of stuff on screen in most mail clients: numbers showing things to be read, names, subjects, dates, and so on. There might even be a warning or two if one of your accounts isn't syncing properly.
I don't know what the answer is. I just know there's something missing.