I think because of the way our brians are tuned NPCs are the most difficult to get right. Human brains are tuned to notice small details on faces, which is why a lot of paper money have portraits printed on them -- because it is more obvious to detect counterfeits. So we are particularly sensitive to NPC faces in games.
I think it's how computers render 3d objects, and they tend to render some things better than others. Cars have looked photorealistic for years. I mean, take a look at this http://www.google.com/imgres?q=epic+games+graphics&um=1&hl=en&sa=N&biw=1280&bih=941&tbm=isch&tbnid=U71tGpzDp_oEnM:&imgrefurl=http://www.pcgamer.com/previews/eyes-on-epic%25E2%2580%2599s-vision-of-the-future-of-in-game-graphics/&docid=ZSASRJ9vOJHRmM&imgurl=http://media.pcgamer.com/files/2011/03/Unreal-Engine-3-man-smokes-moodily-2-627x352.jpg&w=627&h=352&ei=BINKT66zCKe-0QH4v7mWDg&zoom=1&iact=rc&dur=100&sig=113912484493301775241&page=1&tbnh=105&tbnw=187&start=0&ndsp=27&ved=1t:429,r:6,s:0&tx=85&ty=20
(Its blurry, but bare with me) This looks amazing, it has more detail in it than I see in most faces in real life, but it still looks computer generated