So few tweaks would be needed to make Calibre actually user-friendly to the average consumer. Instead people here are falling over themselves in declaring their love for a weirdo UI. Just because it's unusual?
They like real buttons, lacking flat nonsense, and densely-presented functionality. I do too, so I get the impulse to resist calls for change, but agree with you that a light-touch reorganization of some of the buttons & menus, and some filing-off of wholly-unhelpful behavioral rough edges would go a long way. It’s one of those pieces of software I’m constantly nervous using, because I’ve seen it do weird, destructive things for no other reason than some piece of the UI is “quirky”.
This makes me think of Dwarf Fortress. Weird menu system, that becomes endearing very quickly. Emacs too, now that I think of it. I love all three though.