To get the ball rolling, and as an example:
FIX: Session restore puts windows in the wrong place - https://bugs.launchpad.net/ubuntu/+source/marco/+bug/1845822
FIX: USB auto mount with multiple users logged in causes permissions fail - https://bugs.launchpad.net/ubuntu-mate/+bug/1798977
FIX: 19.10 - Panel applet sizing on startup is wrong
FIX: Bad vertical spacing in GTK3.
In short, 19.10 uses about ?15%? more vertical space for things like caja than 16.04 does, from extra padding between rows. This is a real problem on low-res displays like 720p laptops and TVs.
(And the Places panel in caja uses close to 40% more space, but IDK yet how much of that is this bug or if some of if was deliberate).
None of it is deliberate. It's a bug in GTK3, so I expect we're stuck with it forever. It can be worked around for padding values of 0 and 1 only, and is broken for values >= 2. Details are over here: Excessive vertical spacing in caja in 19.10 (and probably earlier)
WISH: Too many other GTK3 bugs to count. Since I only have one left though, probably something to do with getting some amount of color choice back in themes. (See Colour variants of Ambiant/Radiant-MATE themes amongst others).