Result providers were only in charge of computing the pages results, whereas content views also handle rendering of these results, making it possible to control listing layouts and actions available on the listing view.

Also, result providers were difficult to customize, default paged providers only handled document instances and played with different concepts that are now gathered inside a single content view definition.

The blog entry at http://blogs.nuxeo.com/dev/2010/07/content-views.html gives more details about what used to be done, and how content views make it easier. The content view documentation is available here.