For me, the subtle differences/changes that set it apart from the '67 (and older) like the chrome scoop emblems, the '67 had the funky pronged ones and the '68 didn't. The bit wider body style gives the '67/'68 models a better look/meaner stance than the earlier ones. Just having bigger/sportier tires/wheels make a big difference in looks when you put one next to another w/stock tires/wheels (less wider). I also like that Ford began incorporating more and more safety features in '68 like the padded steering wheel and collapsable steering column. I also like that the '68 models don't use shims in the UCAs as in the '65/'66 models. Less things for me to replace and look for!
I do have to say that aside from the GT/CS and HCS my other favorite '68 is the Fastback. I had one back in the late '80s and should never have sold it, still kick myself for it. What does it for me w/the GT/CS and HCS is the GT look they have, w/the scoops, stripes, fog lights, tail lights, etc...
Oscar