Some significant advantages of D5200 over D5100 is
9 cross type focus points over 1 point
39 focus points over 9
5 frame per second continuous shoot over 4 fps
24 Mega Pixcel True resolution vs 16 Mega Pixel
Better and more friendly navigation menu
Of course there are other minor improvements as well that might well make huge differences to a pro, but I myself am a beginner. I will review this question from a beginner point of view. While testing Nikon D5200, I noticed the click is much nicer than the click on D5100. That might be an attraction to many people and I believe it makes big difference in user experience. The D5200 however lags in the same ways as D5100 that in live mode, there is significant gap between pressing the shutter and the picture being. This was something I was hoping would be fixed in the new release but it is not.
Overall I am very happy with my Nikon D5100. Although D5200 is sexy in many ways, keeping the price and looking at the overall satisfaction with my camera, I am not yet tempted to upgrade, at lease for now. What is your opinion?
Check the price comparison below. Listed prices for both the camera is $800.
D5100 ($596 at the time of review. About $200 off)
Nikon D5100 16.2MP Camera with kit Lens
D5200 ($796 at the time of review. Almost full price, no savings)
Nikon D5200 24.1 MP with Kit Lense