I only use GMail as gateway to aggregate my email accounts and synchronize with my Android devices, native mail client.
On Windows and GNU/Linux systems at home, I happily keep using Thunderbird.
E.g. Thunderbird apparently only introduced threaded conversations 7 years after GMail did.
Not all our individual tastes align with what's most popular, obviously.
I still manage my own email servers.
- Speed. It's sluggish compared to the FastMail web UI, and slow compared to Mail.app on my Mac.
- System provided UI editing controls which would bring richer editing and consistent controls
- Consistent hotkeys - I know Gmail has rich hotkeys, but all my other software uses a fairly consistent set which can also access a wider range of keys than a browser can do.
- Automation - I use automation and tools like Alfred (/Quicksilver/Gnome Do/etc) and these can interface with native apps much more effectively through things like AppleScript.
- Drag and drop (you can drag and drop much more than you might think on a Mac, and I use it extensively)
- Centralised notification control in system preferences
- Better (and faster) layout – you're constrained to a web browser so there's less you can do in terms of good use of screen real-estate.
- Prettier – it might sound superficial, but I enjoy using apps that have a nicer design and Gmail, compared with many native clients, looks pretty terrible.
- Real multi-window support - using new tabs doesn't provide the same interface or interaction patterns.
- Real right-click support, with the options I'd expect for any other system app.
That's most of what I'd like to see. Note that an Electron app like Slack doesn't exhibit many of these.