@Zane Matthew[W]hy limit your self to just WordPress? ... Why not make an app store for other open source CMS’?

The WordPress App Store is for WordPress plugins that were made "for" WordPress.

Plugins are "extensions" of the program they plug into. They don't run on other software.

A WordPress plugin (or 'App') plugs into only WordPress, and needs WordPress. Now replace 'wordpress' with 'Expression Engine'.
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Now ... if Expression Engine et al adopt/incorporate the WordPress plugin API, their users might then be able to directly use WP plugins.