well the biggest obvious additions are the new sound device, Thor. it comes with good presets but the fun is creating your own patches. One of my favorite additions is the ReGroove mixer. Forget robotic, cold, quantized music and get with a human sound. You can get tracks with different feels or grooves to have the same groove and vary how much the original groove is changed. then you have the sequencer window. to me the biggest improvement. you can add and view automation, easy to add overdubs or alternate takes. and it may not be a biggie but i love the fact that they added a pre-click to count you in. the only thing the sequencer window is missing is a staff view (but that would probably take away from the simplicity Propellerheads is aiming for.)
there are other tiny improvements but the ones i just mentioned makes the software well worth the $500.