Osama,
Both the quality and the importance scales in English Wikipedia are disorganized. They both were developed in 2004 for this project.
The goal was to sort which articles to burn on CDs and share by mail around the world. As it turns out, people quit using CDs, so a grading system which expected CD versions of Wikipedia articles which were not updated for years (offline versions) never was useful. I would say do not put too much faith in English Wikipedia's grading systems.
Doc James has a list of about 200 articles which he calls "top priority".
Other than that, importance is supposed to be judged by article traffic and links. If many articles link to an article, or if it gets a lot of traffic, it is more important.
No one has imported an ontology of concepts in medicine to arrange the articles into any hierarchy of importance. This would be useful in English but more useful in other languages, but it is not in Wikipedia at this time.
yours,