I like the YouTube app interface, but I noticed that you can't do something that I would do frequently through YouTube on Safari which was that I would play a video, lock the phone while the video was playing (which would stop the video), and I could double tap the home button and press the play button to resume the audio. This would allow me to listen to a song on YouTube while my phone was locked.
You can't do this anymore through the YouTube app though, and you also can't play videos in full screen on the iPhone 5 without the black borders which sucks.