Mine are 1) Xmodmap support/migration 2) breaking imagemagick's import (screenshotting) making my shortcut for OCR'ing text on the screen break.
It seems Wayland isn't going to be the default in Ubuntu 20.04 LTS, but if it wasn't for the risk of Wayland I would be on the non-LTS releases. At least Guix exists now so staying on LTS (released every 2 years) isn't that bothersome in terms of easily avoiding out-of-date software.