Personally, I don't see anything wrong with having the PCs stand somewhat outside the Caste System of a conventional Feudal System, or even to have them buckle the system entirely. Fantasy Genre fiction is full of heroes who bend or break the system by virtue of their skill and daring.
King Arthur's Court is primarily made of powerful warriors whom Arthur chose to recruit rather then banish. Lancelot once battled Arthur out of pride before Arthur recruited him. If your kingdom has a wise ruler, perhaps he'd rather make the strong PCs members of his personal court, rather then punish them for their daring.
And who can forget Robin Hood? He litterally bypassed the conventions of Feudal society by raising an army of brigands and highway robbers to battle his enemy the Sherriff of Nottingham. And yet he's hailed as a hero, not reviled as a criminal.
And everyone knows Conan's handicap with "social conventions"...
I'm also reminded of the "Flying Leaps, Deadly Silks" article from Dragon #289, concerning Hong Kong Cinema. There, they described a rule of wuxia:
There is no such thing as an anoynymous high-level character. Beyond 7th level, your character becomes famous (or infamous) for his prowess and deeds, and word of his activities spread like wildfire.
But you're also knowledgable of other famous warriors, their fighting styles, their masters, etc. This is because Martial Artists (which, the way I see it, can be replaced by the word
Adventurers without changing the subject) live in a kind of half-world outside of normal society. Instead of sticking to your normal part in a rigid society, you've buckled the system to become this being who's physically more powerful then any peasent, yet is lower then them socially because you've removed yourself from society.
Personally, I don't think that PCs in a D&D world should be forced to be respectful to Nobles and Aristocrats simply out of tradition. If the character feels he doesn't need to show a "Weak Maggot" of a noble respect unless he can prove he's worthy of it in a fight... let him play it out. There are always rebels in every culture. Sometimes they change society for the better... sometimes they just get crushed. Either way, it makes for some good adventure material.
