The new UI is still as broken and unfinished looking as it was in the early iOS 7 betas. It still looks unpolished and childish and seeing as the design refresh with Yosemite proved that Apple can mature the flatter UI design language (which they did on the desktop OS) and implement it on their mobile OS.
For example, OS X still uses traditional looking buttons (that actually loom like buttons) instead of plain text hyperlinks which is kinda ironic considering that the normal button style is better suited for iOS since it's a touch-based OS and the OS X-style buttons are more inviting for touch than plain text.
The new default icons on OS X have enough detail and depth while not going completely overboard with gloss and texture and still retain the distinct Apple look. The iOS icons on the other hand, still look amateurish and inconsistent with one another. Say what you will about the old iOS icons but they flowed together on the home screen much better than have since iOS 7. I believe that the iOS icons would look better and much more refined if they took some design cues from their desktop counterparts.
Also, less use of white in app windows. The translucent gray that Apple uses on their OS X windows provides the right amount of contrast while still allowing the content to shine. I think this design choice could translate nicely over to iOS.
I would like Apple to rethink their current strategy with Music to make listening to my music collection an enjoyable and elegant experience (which it hasn't really been since iOS 6.1). Apple should go back to basics here and have the default music app be a simple iPod player again while spinning off the radio and streaming features into separate apps to streamline everything.
While stability and performance are still a lingering issue with the most recent iOS versions, the GUI has been neglected and is starting to look quite stale again and honestly that is a big part of the picture. Apple does not need to do as radical of an overhaul as 6 to 7, having a moderate design refresh for iOS taking cues from OS X seems like a no-brainer and would make iOS devices look more desirable, attractive and functional again.