Someone suggested recently in a thread to allow the player to wear insignia for the guilds or factions they are in. So if you're in a knightly order, people greet you with a little more respect or accommodation.
That's actually a cool idea, have a symbol of office of some sort that displays your membership (and perhaps, your general rank) in a guild, and have people react to it (via things like dialogue, merchant discounts, or enemies hesitating to mess with you, etc). Warrior-type guilds might give you a tabard to wear over your armor, a hood for mages (not the weirdly-done hoods from Oblivion, but something more like http://www.museumreplicas.com/images/PRODUCT/large/1328.jpg, which actually works as a separate item), illegal guilds, like the Thief's Guild or Dark Brotherhood might go for something more subtle, like a ring or amulet, that only criminal types would recognize.