My impressions are:
 
better rendering + (better results, realistic )
faster loading + (the content and the startup)
docking panels + (good)
drag and drop - (this is useless, is better the old double click)
tree navigation - (the thumbs are ridiculous small, you have to click an item to see a descent thumb)
firefly only- (no poser 4 render engine)
lights + (a lot better)
besides this changes, the program is the same.