The battle between UX and UI. UX is all about making something as intuitive as possible for users.
UI is about making something look pretty. Good UI does this in a way that enhances the UX and doesn't detract from it.
The transparency aesthetic here is all about UI and, judging by the launch video, risks harming the UX by making text harder to read. I think if they are willing to compromise a little and make tweaks that enhance the contrast (e.g. increasing the blur applied to background items, apply a shadow to the glass elements to darken the background and increase contrast of icons and text) then this could be a really good modernisation of Apple's software design language and I do like the look, but as it is right now I think its falling a bit short on usability.