I'm just going to assume that 'basic' features like private messages and an advanced post editor with answers, polls, attachments, etc., will be part of the production release or an 'official' extension, so I won't bother to list them here.
The most important feature by a long shot for me is seamless and complete Wordpress integration, where Flarum users are also WP users. Using Flarum for comments and being able to finally dump BBpress would be awesome! I truly believe this would lead to total market domination by Flarum.
Demand a minimum standard of quality from developers building extensions/plugins/etc. Not having this is what really turned me off of Discourse, where I'd go to install a plugin and find that's it's ancient/deprecated and would likely nuke my site if I installed it. Also, many Discourse plugins offer absolutely no description of their features or how they work other than some cryptic GitHub page. Seriously, wtf?
Make installing updates and extensions easy, as in push-this-button-to-update.
Anything else I would expect to get it via plugin/extension.