Obviously, every rule has its exceptions, and breaking a rule can be done to great effect. There's a difference, however, between breaking a rule on purpose and not even knowing there's a rule.
Several things. First off, I'm not sure I totally accept that there are objective "rules" of "good" and "bad" in writting or any other creative area.
However I understand what you mean and I agree to a point, however what you describe does not neccesarily make a writer a bad writer, or a book a bad book, it often simply means they are imperefect, or still have things to learn and room to grow (both of which are true of everyone).
Nextly, I tend to think that most people that even have the inclination to write tend to be aware of the things you speak of or at least most of them. When I started writing I had no formal training at all in it, hadnt even been formally taught grammar, but I knew how things are supposed to sound. Usualy, mistakes in the areas you mention are simply part of a learning proccess. They dont neccesarily label a writer or a work as "bad" and they certainly dont rob it of all value.
Secondly, the reason why there are many different elements that all come together to constitute quality is because sometimes certain elements aren't important in the context of a story
The thing is, which qualities are needed, and even often the qualities themselves, are subjective matters of individual taste.
On the other hand, a lot of the fantasy fiction I read is badly written, plotted, designed, but a few I enjoyed despite it all, and with others I enjoyed them because of it
This brings up an issue we havent really spoken about yet, the issue of purpose, and the relationship of a work's quality to wether or not it fullfills its intended purpose.
The two main purposes behind most creative works, especially I think as far as writing, music and visual arts like painting go, is one the fact that the artist enjoys the act itself (and often has things floating in his head that he wants to get out) and two, to give enjoyment and pleasure to those who read/see/hear/whatever the work. There are often secondary purposes...a writer may want to explore a genre or character type he's never used before, or may want to make a statement..and of course some works have making a certain statement as their main purpose.
However, usualy the main goal is enjoyment.
The two primary goals of actually publishing/recording/selling/whatevering such a work are usualy one, to get it out to more people to do whatever its intended to do, and two, to make money in the proccess.
In the case of Eragon, based on what I have read about it and its author, its purpose was primarily enjoyment; enjoyment of the author in creating it, and enjoyment of the audience in reading it. And then of course it was published to facilitate that, and to make money in the proccess.
It has succeeded very well at both those goals. Many have enjoyed it, and it has made a great deal of money. It attained its intended purpose, which I think its a major contributor to the quality or value of a work.
I think the reason that you and some others are able to seperate "quality" from enjoyment is because when you say quality, you mean level of adherence to a set of guidlines, not inherent value, or attainment of purpose.
Romance novels are in an overwhelming majority really awful and derivative – but they sell extremely well, because their audience doesn't demand Mme Bovary. That's not bad, or evil, or anything – but the books are badly written nonetheless. It's just not important.
Your right. It isnt important, because to those who enjoy them, they are not bad or badly written. They just dont meet your personal standards of enjoyment, which are no better or worse or more or less important or accurate than anyone elses.