I know this message is probably delayed, but here I leave my opinions on the matter:
Note: I'm in no way an expert in MediaGoblin, so I don't know much about how to set it up or whether this things can be done or not; I'm only drawing the cards (exposing ideas). About upload limits: You can also set it even lower, now with Vorbis (and Opus) and Theora (or VP8, or VP9), we can have more light-weight content. Besides, I don't think people need to put something extraordinarely-exagerated-HD if they're not paying for the service. About keeping original file: I, as free/libre software activist would do the following: See if MediaGoblin allows changing the list of accepted files, and accept only those friendlier to free/libre software by default (no .mp-nothing, only those like .flac, .ogg, .opus, .webm, .webp, and those usual text, document, and image formats). And disable conversion. Too bad it's not possible to scan for the exact MIME type of the media being uploaded before it was already uploaded (unless of course we find a way to require the browser to inform the MIME beforehand). Also, perhaps one can set the service in such a way so as to have a limit of uploads for a user in a given week (unless he deletes some of his already-uploaded works, in case for example, he wants to fix something). About centralization: We were discussing this in either #fsf, #gnu, or #trisquel some months ago, and we are also concerned about such problem. Besides, people assume that they don't have to contirbute to the service being provided or that they don't have to contribute to providing an alternative service. An example came from a previous MediaGoblin instance called Gobblin.se, where some user was uploading heavy content to that site, and even some that might be under standard copyrighted license yet. Taking on from the previous paragraph, I also reported (using the "report abuse" feauture/link of MediaGoblin) some suspicious materials being uploaded, that I thought to be under standard copyright licenses. Although it's been a long time since I visited any MediaGoblin service provider (I plan to get back on it and on making videos also). About communication between MediaGoblin service providers: I also hope for that. I saw some talks about it on LibrePlanet 2016 (yes, the one from last year), but the MediaGoblin project's website doesn't state federation as feature yet.
