For me, odd things like inability to have a dark dock and titlebar and light windows. Using quickview on a file in Finder and then pressing shift-up used to select the file and the one above it, and then show a preview of the newly selected item. Now in Mojave they "fixed" it so that it shows the preview of the first file only, which is a colossal pain because now I can't filter through a giant list of photos to decide which ones to keep in an easy quick way. I have to look at each file individually.
I am sure I will think of some other annoyances. I've kept a 2012 MBP on Sierra, and love that machine far more, eg. keyboard is actually good and is quiet, real function keys, real ports as found on every other device in the universe, ability to use my audio interface, network ports etc etc
A couple of control freak things. I despise the junk mail folder that can't be hidden or removed and requires an extra "are you sure" click to delete messages. It refuses to copy files from my external hard drive. I have to disable SIP to do some fairly normal stuff. Siri, though I have it turned off. Location on photos.
