Some main things are the use of gussets (triangular pieces of material under the arm) and gores (triangular skirt pieces for girth on the sides). Also the sleeves have not simply been hemmed, but sport actual cuffs which is an easy way to add a nice look to an outfit.

I chose the dark blue lining for the collar even though it was not intended to be seen, for the slight contrast it gave. Always be open to 'thinking outside the box' to make your costumes more unique.