Reverting to old UI
There is one key feature that is missing in 1.4.0, in my opinion: the ability to use the good old UI, which indeed hasn't changed for years, but for several good reasons.
The new interface makes Gajim look more like WhatsApp-Web, but it misses on many advantages it used to have, such as: (i) the ability to have multiple chats open and visible on the screen, detached from the roster window, (ii) proper alignment also for RTL languages, (iii) the ability to navigate easily with the keyboard, (iv) the ability to see all contacts from all accounts on one list, etc.
I personally ended up re-installing version 1.3.3, but in my opinion there should be a better way to keep enjoying the core advantages of the traditional Gajim UI.