YouTube
I have iPhone 3G and I find the YouTube app pretty useless. Maybe I'm doing something wrong.
Videos take ages to load, if they load at all. Most of the time it gets stuck and won't load any more. The skip back and forward buttons are annoying. Even though I have verified my age and sign in to my YouTube account, I still can't play age restricted videos.
I don't think other phones have the problems I have when accessing YouTube.
I have these problems over a WiFi connection. It's pointless using YouTube over 3G for me.