I have 450+ friends on facebook, many of which are acquaintances, or just people I don't care to have on my phone. Wouldn't it be great if we could selectively sync by leveraging facebook's list feature? I'd like to just have my "close friends" and family show up on my phone, or something like that. It would also be nice to have list-based filtering in the contacts app.
This may also be a good idea for Google+ circles, Google Contact Lists, maybe even some other services I'm not thinking of.
Post here if you'd like to see it happen, I know it would certainly improve the useability of my phone.
EDIT: I guess this should be moved to the Synergy board... whoops. Do it up, mods.