You can host media files on Wordpress or Drupal, but the goal here is to build an application that's very focused and tailored user-interface wise to media. Furthermore, Wordpress and Drupal alone might not very easily handle some of the extra tooling that MediaGoblin does: if you upload a video, that video needs to be transcoded; we have task queueing and infrastructure so that these things can happen in the background without you just sitting there and your browser timing out.
As for how federation will work, we may enable a plugin called "diskgobbler" or something that might dowload files, but no, by default, MediaGoblin will not mirror all files from the other sites it is federating with. :) Instead, the goal is to keep the social side of things alive: you can create galleries that have entries that span multiple sites, favoriting a piece of media can happen across instances, you can subscribe to things a-la YouTube channels. That's what we mean as in terms of what we aim for by federation.
As for how federation will work, we may enable a plugin called "diskgobbler" or something that might dowload files, but no, by default, MediaGoblin will not mirror all files from the other sites it is federating with. :) Instead, the goal is to keep the social side of things alive: you can create galleries that have entries that span multiple sites, favoriting a piece of media can happen across instances, you can subscribe to things a-la YouTube channels. That's what we mean as in terms of what we aim for by federation.
Hope that answered a bit!