I use my phone as an MP3 player at the gym, and I like to change out the music regularly so USB 2 speeds are paramount for me...beyond that, it'd be nice to have QVGA screen resolution and WM5.
All phones I've looked at seem to have USB1.1 :x
It'd also be nice if whatever phone it was was a "supported" phone...meaning that the mfr gives some regular updates to include things like stereo-over-bluetooth ... that it's not an 'abandoned' phone.