I find statements that Mangini did not bring anything new to the table a bit odd. Mangini brought a new drumming philosophy to DT, which is melodic orchestration. Take Dystopian Overture for example. The drums and cymbals are played melodically, not just rhythmically, if one pays attention to the tones of the toms and the cymbals. It really goes up and down the scale melodically. That drumming philosophy of complementing and integrating with the other instruments underlie his playing, not the technical stuff. The technical stuff is deployed only if it fits the orchestration, like the use of polyrhythns in The Walking Shadow and Illumination Theory, or the use of speed in The Enemy Inside and Moment of Betrayal. There really are less air drummable moments for MM because the drumming is focused on orchestration, not on fills.
Another thing MM brought new to the table, as Jordan said, is that he helps in thinking how to subdivide sections. In the Dream Theater album, there are lots of sections where the different members are playing seemingly different things at the same time but ends up integrating in the end. You can hear such sections in Enigma Machine, Illumination Theory, Surrender to Reason and Behind the Veil.