Page Previews have issues on many projects. This epic will track the changes and project-specific implementations. Depending on the project, previews will either contain the current implementation, a project-specified implementation, or will not have the feature altogether.
Wiktionary - appear blank or only display image
Wikispecies - display images only
Wikiquote - seem to be working fine
Wikibooks - can turn on in beta, but doesn't make sense
Wikisource - don't seem to work
Wikinews - blank (potentially because there's no lead paragraph)
appear for articles with lead paragraph
Wikivoyage
blank or strange for certain pages
Everywhere else - we should probably disable