I noticed it in the Buffyfandom the tendancy to have Angel do no wrong, while Spike could do no right. The total hero worshipping of one character, while constantly pointing out the flaws in the other. The whole point of character development in programmes is that they do develop. Spike fans can see his flaws, and also acknowledge them. We know where he started off, and what he became. It was what made him one of the most interesting characters in the show, and was what made Joss such a great storyteller.
Now in SPN we have the same thing with Dean can to no wrong, Sam can do no right. While the character development of both Sam and Dean is not as well written as it was for Angel and Spike, especially Spike (Angel could be very boring at times, he was quite static), the fans reaction is the same.
One character who is constantally good is not good storytelling. While Joss is better than Eric in that field, there has to be some movement in character. Yes EK may concentrate on Dean more, and show us his developements in more detail (another example of bad storytelling, after all he only had two main characters, so could've spent more time showing Sam's motivations last season, a major flaw of the season in my book).
If one character can do no wrong then they are boring, which I don't think either Angel or Dean are, it is just the depth of the character is being ignored. Spike, for example, moved from being almost a pantomime villian in Season Two, to a fully developed souled vampire in Season Seven. One we understood. While Angel was always far more interesting when we got glimpses of Angelus. To see where he started in his full evil character, to his search for redemption with his soul is what makes him interesting.
In dismissing the character to this one dimensional always right, always wrong way takes away any depth, or humanity (yes I know two of them are vampires, but they have souls). Humans are flawed, and so should good fictional characters be as well.
The depth of character development shown in BtVS and AtS, of each and every character in the show, is what makes Joss such a great storyteller, and his keeps his fandom alive. We just know we can put these character in situations and explore how they would react. I'm not sure that EK has given us enough information on either Dean or Sam to be fully inside their heads they way we can with Buffy!Verse characters.