Most “missed opportunities” I can think of relate to product design or functionality i.e. features that were needed or expected but that didn’t exist out of the box such as easy theming, lightweight analytics dashboard, grade book, live conferencing tool integration, chat and messaging tools, etc. These are features often found in the more “traditional” LMSs, and users expect them. Of course there’s always the possibility of developing such tools but it comes at considerable cost.
One could argue that Open edX has serious product/market fit issues - however good our marketing activities can be, this will remain a core issue. But let’s not forget that it began as a MOOC-centric LMS (and still is for the most part) and that it’s slowly evolving towards something larger and more permissive.