IMO, the main issues with the UI isn't any lacking beauty/animations (and please don't add any in case it slows down the program even more), it's the inconsitency. Let me explain:
By default, Calibre users mostly use/see the main toolbar and the context menu. However, I've found that there are several overlapping functions that appear in both the main toolbar and context menu, leading to confusion and waste of space. Every time I install Calibre I have to manually reconfigure the UI to my liking (I now don't even use the context menu). I wish there was a way to backup/transfer the UI settings to a new installation so I don't have to adjust/remember where all my UI preferences are.
Furthermore, is there an easier way to move/adjust the UI elements besides within the Preferences menu (preferably drag-n-drop, like Google Chrome, which is an epitome of UI design)? It's very tedious to have to use the Up/Down/Left/Right arrows to move the icons around within the toolbar and context menu.