On the outside as Doug says it's just cosmetic.

There's a few updates on the inside (power supply, CPU, RAM) but nothing that drastically changes functionality other than the double speed MIDI port mode has been removed. Seeing as the Polaris was the only synth to have this feature its not a huge loss unless you were connecting two Polarises together (and you can't use a second Polaris as a polyphony expander anyway). I'm sure Paul will chime in on the finer details of the internal changes...