-It was the middle ages, they didn't bathe but once a month or something like that. Arthur may have bathed something like once a week, and the servants of royalty may have been forced to bathe more often, but on the whole they didn't bathe as regularly as today.
-Arthur was raised to believe magic was evil and wrong and purely just bad, so I doubt he's going to roll over and be all happiness and flowers when Merlin's secret comes out.
-Religion was a big thing back then, although it's not really portrayed in the show, the church was around as was all of their menacing influences.
-Chivalry should factor in to Arthur's behavior sometimes. I mean they weren't Knights in shining armor for nothing.
-Also, as a whole the middle ages were a lot grittier, dirtier, meatier, rough, and more raw than what is portrayed on the show.
There are probably more things to add to this list, but this is just something that I was thinking about while reading fic.