The population of a forested kingdom never reveal their names to any but their immediate family, and consider it a grave mistake when others do so.

A culture must hide its hands in the arm of their clothing as a sign of respect and peace. When approaching somebody you show them respect by crossing your arms and hiding the hands in the shirt sleeves. Nobody worries about hidden knives and such, it is the threat of magic that this custom was created to prevent.

It is said that though the crown has always been passed through inheritance, no man may rule without the approval of the Archmage and the Guildmaster. (Interpretation I leave to interested parties.

"...The people of the Dalaben Fens have a custom heretical to our superior Zodashan faith, in which they place their dead upon the grass roofs of their stilt-houses. It is said that they do this because they used to bury their dead before the great Judgement, in which their home became the swamp which it is today."-Sir Edrosh Metorva, "The Barbarian Lands of the East, Volume X."

The ruling family depends on some magical artifact or place to secure their right to rule. A monarch isn't considered legitimate until he or she has been "accepted" by the artifact or place, whatever that might entail. It is also possible that some one else, not of the ruling family, could be accepted as the legitimate heir... A fact which the monarchs would like to keep secret.

Throw in a handful of random customs - like women or men of a certain level of society being required to wear a veil or a pendant of some type.

When you sleep, your soul briefly returns to the place where it was created/goes to when it dies, and seeks nourishment. Everybody's soul is created with a little link (or you could say a greater entity calls it back) to that place so that it can return easily, but upon death, the soul returns via that and annihilates the link as if returns.
Also, as an added thingy, you could say the entity supplying the link, or residue of the links themselves, (whichever) is the source of all arcane power.

Marraige custom. The groom must prove his worth to the friends, family, and future wife. The best man is obligated to challenge the groom to a duel and the best man chooses the weapons. (can be anything, toothpicks, cotton balls, anything) If the groom wins then he proves his worth. If he loses then the bride will accept him for his faults and could sometimes gives him a task to accomplish before the wedding.

Facial tattoos in various designs are a sign of status in some cultures.

A culture believes that souls are recycled. One child gets half of a soul, another child gets the other half. However, this means that with each passing generation, the amount of soul in the child will become less and less (Through division)until their culture will die in the future because of their soulless offspring.

Minor Cultural Modifier: The culture believes, for the soul to leave the body, the corpse' eyes must be open. So any member of that culture takes care any dead body they find has its eyes open. Doubly true for those they kill themselves, for then the corpse will surely rise as an undead hunting him.

There is a group that believes no good can come of trying to change to future to render a prophecy or prediction incorrect. They believe that meddling will either cause the prediction to be true in the first place, or make something worse or more extreme happen.

A culture has a tradition of wearing animal pelts as a sign of status or job. Carpenters might wear beaver skins, Masons have a moleskin hood to their cloak, Gaurdsmen might have badger pelts. Done to show the culture's respect for nature and how much of nature is equal to each other.

Atheists and agnostics get a jolt when one or more deities make an appearance, and take a direct interest in things. It is hard to deny the existence of gods when one has met them face to face.

Bards and minstrels, if business is slow, will moonlight as freelance diplomats, couriers, or spies.

The army relies on a loosely organized, semi-freelance corps of scouts for information-gathering. The officers in this scout corps are usually retired spies.

A certain culture believes that evil can live beyond the grave. They also believe that the destruction of the dead body can keep that one from coming back. Destroying the newly deceased body can destroy the soul if done properly. They first burn the body for 24 hours, smash the remains as well as possible, then burn the what is left for another 24 hours.

Diseased people, (leprosy or any other such fear inducing disease) when traveling, will often times wear a cloak that entirely covers their body and ring a bell as they travel, used as a warning for any others to stay away. Could be used for a disguise or safe passage.

A people who believe it is incredibly impolite to speak to anybody they meet for the first time. They believe their actions should speak for them until they are comfortable in each others presence and can then trust each other. Only then would converstation be appropriate.

During battle/war the heads of all slain are decapitated and collected by the slayer. The slayer will clean them up and put them on display for inspection by the commander. Depending on the deads name, rank, and expression on the face held by the death blow-the slayer will be rewarded by gold and/or status.