Higher IQ at higher ISO and better dynamic range......and keep my DX lenses.....color=blue>
D5200 will have better high ISO quality depending on which camera you are coming from, probably aside from the Nikon 16mp DX cameras which are already very good, and which are basically a draw with the 24mp sensor as far as high ISO is concerned. It will be interesting to see if this pattern holds true for the D5200.
According to DxOMark (and using the D3200 sensor as a stand-in), the D3200 has equivalent ISO to the D5100 and beats the D5000 (1131, 1183, 868). Of course this compare tool can also be used to call up even earlier sensors which should all be beaten rather handily by the D5200. The other thing to keep in mind is that the D5200 14-bit compressed RAW will offer some further improvement over the D3200 12-bit RAW IQ but am not sure if that will affect ISO the rating:
Anyhow, I am a gobsmacked that Nikon included the 2000 pixel RGB metering sensor AND the 39-point AF system from the D600/D7000 lineage. Combined with the 24mp sensor, I think this is a very very nice update indeed! I believe it will be a great camera, very nice package of features.