The answer is: Allowing you to easily find certain types of people over your hundreds or thousands of contacts. Types = certain business or personal relationship, certain skill, certain place you met, etc.
Here are the core features, the minimum viable product. In fact, I would avoid something that is bloated and doesn't focus on these core features:
* Automatically importing and deduping your contacts from a variety of sources: email contacts, linkedin, facebook, maybe twitter.
* Automatically assigning a rank score to each friend, based upon your frequency of contact with them, and analysis of the social graph. You want your spouse to be ranked high, and the guy you met once at a networking event to be ranked low.
* Depending upon interactions with people, you can upvote or downvote them, to change their rank score. Ran into someone you want to hang out with more? Upvote. New girl made a date with you and then stood you up? Downvote.
* You can search or browse by tag. There is smooth autotagging and query expansion, to make sure searches don't miss certain friends.
That's it.