While being able to replace The Loop is a big step forward, it will be of limited benefit if the new content uses a completely different structure than the theme. The result will likely be unstyled and will look out of place.
The final piece that ties everything together is a HTML and Class structure that establishes a foundation of styling that allows plugin content to look like it belongs on the site.
Themes that implement the Standard Loop can use add_theme_support('standard-loop') to indicate that they follow the standard. This allows plugins to use current_theme_supports('standard-loop') to determine the support offered by the current theme and take action as needed if it is not available.