Spent lots of time perfecting the difficult face of one of the most iconic fictional characters ever. Darth Vader has always been a very hard skin to make, and despite the large amount of attempts, only a few pulled it off.
Reasons to this is firstly because Vader is one of the many characters who would benefit from an odd-numbered amount of pixels horisontally. Also, it's black, and dark colors are harder to shade than others. And then his armor is made of different materials as well, which makes it even more difficult to show that everything is made of different stuff.