My analysis is that the 15 Mustang carries the torch forward with some new features and technologies, along with refined engineering, but the package stops short of "groundbreaking" by not increasing performance to the degree necessary to stay up/eclipse the competition. For those reasons, among others, Ford doesn't offer marketable reasons for Mustang enthusiasts who own 11-14 Mustangs to trade up, especially the Shelby owners. The move to the 15 Mustang makes more sense for those who own older Mustangs (03+04 Terminator owners not withstanding) or are new to the Mustang arena.
For my situation, the 15 Mustang doesn't offer me one compelling reason to take a depreciation hit to trade into a 2015. Besides, I like my "simplified" interior better!
2014 Mustang GT Coupe Premium. Options include: California Special package, comfort package, navigation, glass roof, shaker pro system, 3.73 ratio limited slip axle, GT front grill/fogs, MGW race spec shifter, BBK X-Pipe, Corsa GT500 Quad Sport axle-back, Ford Racing ProCal tune. "If I wanted insane HP, I would have purchased a 2014 Shelby GT500!"