The main differences has to be that
a) both 747 and 757 have their own 192kHz DAC audio decoder that allows them to play full fledged DVD Audio tracks
b) The 757 has a 12-bit/108MHz DAC video processor, while the 737 was 10-bit/54Mhz.
In addition to this, the 757 can play almost every media you can think of: DVD-Audio as mentioned, SACD, CD-R, Mp3... you name it.