I don't think that saoir is particularly talking about UI aesthetics or the list view of the database but rather the technical level of some of the screens.
The main technical complexity is on the conversion screen, but this is necessary for the level of conversion control available. Perhaps a "Quick Convert" button could be useful for non-techies. This would simply convert to the user's preferred format using default options. Although this would only save 1 button press it would at least hide the conversion option dialogue.
There are also some technical terms which could possibly be simplified: e.g. "Edit Book Metadata" could be "Edit Book Information".
Personally I'm very happy with the UI and maybe most calibre user's are quite eBook savvy. Other users may be well served with the software supplied with their reader, so overall it may not be worth the programming effort to add a simplified UI.