Thank God, I've been waiting on a thread like this for ages.
I've given a lot of thought into this particular subject, and have concluded that a protagonist does not have to be likable in any sense for the reader to root for him/her. IN FACT, if the author is really good at what he does, he can make a complete bastard of a person that people still want to see.
It's not as if they want the character to succeed, rather they want to see the character's story succeed. Remember, a character with no redeeming values can be written as interesting, but a good author can write an evil character as a more interesting plot point than a good character.
Lately I've been seeing many authors of books, movies, and games taking the easy way out by putting in a good reason for what they do. Whether it be that the Villain Protagonist's family was killed by a gang or his town razed to the ground by even more evil bandits. While this is all fine and good, I personally would find it more interesting for the story to be about that gang's leader, or those band of bandits.
I've concluded that a story such as this would break up the flow of Villain Protagonist tales, if written properly. On the opposite end of the spectrum, how would a story where both the main protagonist and the main antagonist are completely good with differentiating opinions?