I find it odd at how much of the reasoning isn't merely tailored to RPGs. Great music, character design and engrossing stories that are in the manga style could essentially be inside a number of genres. RPGs just seem to be the only genre to typically cater to those ideals. What's really odd though is that gameplay has shown up so few times thus far as reasons.
Even if you look towards turn based strategy RPGs, this becomes even more clearer. Look at how many RPG fans get into them, and then don't typically get into the other TBS games like Advance Wars and Civilization that feature the same gameplay, just without the manga overtones.
I really think a lot of RPG fans (I don't mean anyone in particular or every fan either) really don't enjoy the gameplay all that much and merely tolerate it to get to the other aspects of the game they enjoy.
Can you really think of any other gaming genre where a commanding lead in appeal isn't the gameplay?