Another usability issue throughout Mac OS is useless key-combinations. One rather small but impossible to figure out without help is reordering of menu-icons. Instead just drag & drop, you must press cmd-key down while doing it.
But why? Why just not simple drag & drop?
Mac Problems 
So you don’t accidentally rearrange them. I really don’t see a usability issue here.
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Do you accidentally rearrange your Dock? Well, i do rarely and it is easy to fix. I count this one usability issue because there is no way of knowing how to rearrange title-bar services without Googling. Drag & drop on other hand is intuitive and used in everywhere, even on other portions of MacOS (like in Dock).
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