I think the basic concept stems from the developer, Kris, making Marvin (and other apps) because he enjoyed it rather than because he was trying to make tons of money from it.
- There was no subscription plan. He had a 1 time purchase of $5 and had some basic functions if you wanted to donate more...basic things like color themes, nothing that changed the functionality of Marvin.
- You always got a fully functional app, nothing locked behind a pay wall, for testing the app.
- The app supported the latest standards: most of ePub3/CSS3 that was available at the time.
- The app did not lock users in to a walled garden to maintain/download your books. It was open to a greeeaaat many sources like: OPDS, Dropbox, Cloud picker, web browser, itunes drag and drop...
- There are a TON of settings to personalize the way the book is presented, not just font size and margins....what items show up in the menus, what is displayed in the headers/footers...how much room is taken up by the headers/footers... color/texture themes...page turn styles...how the author and book list are sorted/displayed...geotagging...passwords...kid locks...
- other reading experience options...web lookup of words/definitions...brighten/dimming plus quite a few other gestures...
- custom menu creation...
- highlighting
- bookmarking
- exporting
- syncing (although there are some that will say it didn't do everything THEY wanted it to do in this area...no matter how specialized their request)
- text-to-speech with multiple voices/languages
Just tons of options to suite 99.8% of the users out there (there's always the .2% that are never satisfied)
Aaaannnddd...the coup-de-grace .... 98% of Marvin still works just fine! (Even if you can't get it on the Apple store) Not many apps continue to just work even after not being maintained in a while.