Given how much time they had, the show was really quite good at developing the characters. When I read the books, the biggest thing I took away regarding GURM's characters was that there wasn't anyone who was purely good or purely evil (unlike most fantasy novels); everyone, even the "good" guys, had a dark side to them. Conversely, even the most evil characters (Joffrey being one of them) had a good or tragic side to them; in Joffrey's case, while he certainly isn't likeable, there is definitely a tragic aspect to his character. Here's a person who has been so misguided, both by his parents (real and imagined) and those surrounding him, that he has absolutely no clue that his actions are nothing but harmful to everyone else.
As for why we don't really see this in the show, it's becauser the books are written from the points of view of the main characters themselves. It isn't quite as easy to get in the heads of the TV characters as it was in the books.