The biggest feature would be adding OS X/NeXT services/Android intents: currently it's impossible to have two apps interact without the developers explicitly baking in support. Allowing applications to establish some conventions for services ("Open web page", "View PDF", "Retrieve login information", "Subscribe to feed", "Post to {network}", etc.) would make the massive improvements from the integrated Twitter/Facebook support available for to all apps and services.

This would also be a smart move for Apple as de-privileging Twitter & Facebook would mean that they wouldn't be helping support the lockin for those services – if Google Plus came back from the dead and started eating Facebook's marketshare, Apple would benefit without changing a line of code.