Wikipedia:Defending article quality
Wikipedia's editorial model and the Wiki way support change and boldness in editing. However, it has been observed by some Wikipedians that there sometimes comes a point where new bad edits begin to outnumber new good ones. As total edits to an article approaches some number N, quality increases (along some unspecified function); from that point on, quality tends to level or even drop with further editing. Therefore, when an article has reached this quality plateau, further editing should be subjected to greater scrutiny. This is called defending article quality.
This phenomenon has been observed most often on featured articles. The reason for this is that featured articles are held to a fairly stringent set of quality criteria, and new contributions to FAs often fail to meet those criteria. Though any article can theoretically reach the quality plateau, the Wikipedia featured article process has been judged by outside sources to be fairly rigorous in picking out Wikipedia's highest-quality articles. 
As such, featured articles should be the first place to start when defending article quality. We should look for bad edits on all articles, and use featured article status as a guiding light to where we should look; however, no one is advocating that edits to featured articles be reverted, or that featured articles be protected. This proposal will merely suggest guidelines for assertively patrolling featured articles and examining new edits with a critical eye.
Visibility and editing
Featured articles, due to their increased visibility, are greater targets for vandalism and something referred to as egotistical editing: when users make useless and/or arbitrary changes just to "make a mark" on the page and its history. Edits on featured articles that seem to have no reasoning behind them should be regarded extremely critically.
Make your case
Make the case for your edits when editing featured articles. Use edit summaries (or the talk page) to assertively explain why your edits help the article and why your version improves on the previous one. Of course, you should always use edit summaries to summarize your edit rationale, but on featured articles use them to outright argue for your version.
Possible solution: Featured article patrol
To combat this problem, a page (perhaps several, due to the large number of featured articles) entitled Wikipedia:Featured article patrol should be created. It should contain a list of featured articles, and users can list their names under featured articles to signify that they are patrolling those articles. Each featured article should have a number of trusted users listed under it.
Solution: Flagged revisions
As of 2008 (several years after this essay was originally written), revision flagging is implemented on the German Wikipedia and will likely see some form of implementation on other languages. This would be a valid solution to the problem, by allowing stable articles to have new changes screened in some fashion.