Feature summary:
An article that is in the main namespace and that does not have any non-hidden categories (including categories added through an infobox or other template) should automatically display a message at the bottom of the page. The message should encourage people to add categories to the pages. It should be configurable per namespace and per wiki (e.g., the message appears only on uncategorized articles at the Wikipedias, but on uncategorized Files at Commons, and nowhere at the Wikivoyages).

Use case:
Wikipedia articles need to be in categories, but sandboxes and other drafts should not appear in categories. This means that the article's first author is usually creating the page, moving it to the mainspace, and only then able to add the categories.

Other editors are watching out for uncategorized articles. 94 Wikipedias and some non-Wikipedias have a copy of Template:Uncategorized, a template that some editors add to new articles. Adding this template takes their time, and it often causes edit conflicts, as the article's author may be trying to add those categories just as the other editor is adding a warning message about the need to add categories.

Benefits:
RecentChanges and New Page Patrollers would have less work to do. Article creators would have fewer edit conflicts to resolve.