When Boeing announced the 717 program, the company made 13 major systems changes (and untold hundreds of minor changes) to the MD95 design. The two that stick in my mind were an upgraded APU by a different manufacturer and replacing the entire VSCF electrical system (constant source of problem in MD90s) with a more traditional IDG generator system. Hydraulic Transfer Pump was replaced with one about 50% larger (the "weak link" in MD80/90 hydraulic system). There is more, but those are the only ones I recall from the Boeing publication some years ago.
*NO CARRIER* -- A Naval Aviator's worst nightmare!