GNOME (by default), Snaps forced on you OOB rather than being an optional thing during setup or in any of the GUI settings, locking newer security updates behind a subscription, and Cannonical just being a bad captain of the ship.
There’s a reason why Mint is hailed as the new “casual (non-gaming) user favorite”, since it’s literally just Ubuntu sans GNOME and the hostile update design decisions.
As a youngblood I have heard of the iron stability of Debian, but simply find it difficult to adopt for gaming-related reasons (both hosting and as a game client).
I do wish to one day study the greybeards’ work for a future project box, however. That knowledge needs to be part of my generation as well, for at least a few of us.