So by calling someone a wang face, you're pretty much telling them they have the ugliest face you've ever seen.
It can often be used humorously as well, just because it sounds funny.
Hehe. wang. hehe.
also can be used to replace wanker.
xxkittykatxx: Ugh, no! wang face!
Tracy: Hahaha, did you see that wang face that was hitting on me this morning? What a fucktard!