Warning: This is not a Wikipedia oficial guideline and it should never be used as an strict manual. It is only a reference for me.
Text is well organized edit
This first point states that the article should be attractive to the eye and have all the pictures in order. No matter how an article is written, if it has no order in its images or a logical sequence, the article may give an impression of a very bad written one. This is very common in Class-B articles because disorder on pictures or text may be an advice that is passing throgh a major edition.
The article is written pretty nice edit
Maybe this is the most important point of all. An article must have good spelling an punctuation. Also must have an order in its ideas (yes, as it has been noticed, this point ranges all characteristics of a Featured/Good Written article. Bad written style is a charateristic of a stub.
Has tables that support the article edit
Sometimes, tables are very useful in an article because they synthesize all the information. Why this? Because there are many people that only need quick data and not all the information written in the article. But sometimes these tables may drive the person to read all the information in the article.
The pictures support strongly the article edit
This point is appliable only to articles that need pictures. And, for those article that need them, pictures are very important because they give an idea of what are you talking about. Also, pictures illustrate examples of some situation that is not enough to describe with words.
edit
Templates have two functions in an article; tye first one, is to synthesize (as tables, but more attractive) information in the article; and the other one is to link the page to related articles. Sometimes pages that have no tamplates suffer something that I call "page isolation". Page isolation is the effect that occurs when is very hard to access to a page (that has no links to related articles or templates) and hence to contribute.
The article cites its references edit
References makes an article to be believable. When an article does not cite its references, it may be not true, an then (sometimes) comsidered as an original research, that Wikipedia does not accept. So, every time you contribute with a fact that has been comprobated, try to cite its references.
It has an introductory paragraph edit
An article must have a paragraph that explains of what its talking about. This is useful because it gives an idea of what are you reading. Generally, articles that have no introduction, has not so much quality and confuses more than a Featured/Good Article.
Stability and NPOV edit
Another important point that makes a GA/FA article. An article article must be in neutral point of view More info here always; because the content is not only for a group of people that think of that way but for everyone. Stability is linked with NPOV, and is when an article is not edited so much (most of the times due to edit wars) which guarantees the article will be kept in those conditions for a long time.
The extent of the article edit
Doesn't matter. Sometimes very long articles may break some (or all) of the really important points (listed above).
But must important of all, write those articles in good faith and wisely.