99% of the time, my phone is in my pocket or the screen is off, so I don't really care about the UI, as long as it's usable (I'm looking at you, the Google Play Music app).
I like the UI right now and think it is quite intuitive and easy to use.
What I miss the most right now is a Chromecast support (the audio and the video versions), but from what I read, it seems that a basic support is planned, so great.
Except that, I'm mostly interested in new "small", quality-of-life, features (like those listed on the first page).