Let me add my friends and show me an aggregate of their favorite and/or recommended apps.
I'm much more likely to use and enjoy apps my friends are using. I'd like to see a list of their apps, especially ones that are popular among them.
Neil Sedger commented
Yeah, there could be a 'friends' page where you see a list of all apps all of your friends have.
It could be sortable by popularity (among friends) or date of install, and filter options for apps I have/apps I don't have.
Later enhancements could add app promotion - each user is allowed to promote say 1 app per day/week, this will be highlighted on the list and optionally notified by email/facebook/twitter.
And then comments on an app so you can discuss it with friends.