i disagree that its over defined except for the zip, i'd make that a little less apparent.
I think the folds need some work though, my fav ones you've done are on just the shoulders, other than them im not really digin them that much, remember to think about how leather folds, its thicker than cloth and more rigid, that makes bigger stiffer folds, also remember the key parts where folds occur, folds are gathering of material. You have to think about where tension occurs and where the material will hang of it, like the back, the back will have little/ no folds at the top because the material stretching across the upper back because its hanging off there, thats where its supported from, but further down as it hangs the cloth hangs and and it gathers at the bottom in mostly vertical folds (like a curtain). However this is effected by posture and movement, twisting the body will make folds appear diagonally from the shoulder to the opposite corner mostly. Anyway, main points where folds occur in a leather jacket is around the elbows and the armpits. meh, folds are unbeliably complicated imo, i still hate drawing them... maybe this tut will help you:
http://www.cablenet.ne.jp/~japanime/tutorial/clothingmain.html
didnt see teh update >_<, it looks nice but shaddows should be less saturated than the main colour unless in percific lighting, this is why using multiply brushes can be a bad idea because they add to the saturation. The most saturated part of the skin should be the point where the light transfers to dark