It's the paradigm, the absence of automatic sorting of emails (priority/promo, etc.), failure of prioritizing common actions in the UI (everything is the same, the spam folder and the inbox folder are given the same importance, "compose" is not prominent, etc.). I could go on forever.
IMO, it's kind of how good software was made 10 years ago, but now we're used to much better.
Not sure what priority/promo is, but Thunderbird certainly can sort by any field by default. It sounds like you want a client that is tightly coupled with Gmail's features, and you're not going to get that with a non-Google client.
Note that some customizations, like Firefox, require accessing about:config. Also, as with Firefox, most Thunderbird users will benefit from perusing extensions. I wouldn't be surprised if there are some extensions that offer the more Gmail-centric experience you want.
> It sounds like you want a client that is tightly coupled with Gmail's features
I actually don't like Gmail that much.
> you're not going to get that with a non-Google client.
Google Inbox used be called Sparrow (https://en.wikipedia.org/wiki/Sparrow_(email_client)), which was a regular app that Google acquired. I used it since the beginning. It was a new paradigm back in 2011, which is now the standard in modern clients.
Thunderbird needs to rethink their approach. They need less buttons and more focus on important features over other ones you barely use. In Thunderbird, everything is given the same importance in the UI. There's a toolbar with 100 buttons of the same size: "Write, Chat, Reply, Reply all, Archive, Address book, Get email, Tag, Quick filter". Why is "Chat" the same size as "Write", and right next to it like it's the most common feature in an email client? Why is "Address book" even there, given the same importance as "Write"? Who browses his address book randomly? Why can't they just show suggestions when I'm in the compose window and I type the first few letters of the person's name or email?
I could go on forever.
My main Thunderbird toolbar has get mail, write new message and quick filtering. It took me perhaps 10 seconds back when I first installed it on this computer to hide a couple of other buttons like chat that I don't use myself.
The second toolbar when I have a message open has just 5 buttons (reply, reply all, forward, mark as junk, delete) and I use all of them frequently.
I agree with your general point that clean UIs that prioritise more important features tend to be nicer to use, but I think you're dramatically overstating the problem here.
Why can't they just show suggestions when I'm in the compose window and I type the first few letters of the person's name or email?
That's exactly what it does, and has for as long as I can remember.
Could you name a few apps that have the Inbox/Sparrow features like snoozing an email until some time in the future?
I would love to have that, I looked for Thunderbird plugin for that but failed to find one.
