This thread is practical, rather than theoretical.
As a freelancer, I work on a number of community platforms (currently Drupal, vBulletin, Discourse, BBPress and Facebook) and I find it increasingly frustrating when I can't use a feature on one platform that seems like such a fundamental aspect of another.
Here for example, I find it frustrating that I can't @tag people, 'Like' a post, or PM.
We often discuss the suitability of different platforms, and I think the first place to start when making that kind of decision is at a really granular level.
What features do you consider key to community engagement?