One that is nothing but positive: you can edit post titles and use limited markdown in them.
One that’s done as much good as it’s done harm: polycentric moderation. One instance can’t enforce its own community rules on others. It protects lemmy.blahaj.zone from bigots, but it’s also why Lemmygrad exists.
Somebody made it this way years ago and there’s not enough demand to warrant the effort to change it. KDE Plasma is 16 years old. GNOME is 25. Some features are so deeply embedded in the spaghetti code that any significant change would result in a cascading break.