Mostly that rdio does not display full screen yet the touch input maps at full screen so it is unusable.
Admittedly, I have not tried it with an iPhone 4 but based on the comments here, the interface is a snappier since it is not using bluetooth for touch input.
It would also be nice to have fully functioning Appradio Extensions, even though mikedecaro seems like he has been working tirelessly on updating it, it still doesn't fully work.