Refer to travel books on Nepal for details on day to day street-level guide on cultural issues. The list here is just a simple, brief introduction and in no way exhaustive:
a. Use your right hand to eat and deal with food. Nepalese use their left-hand to wash themselves after defecating.
b. Once your lips have touched a food item or its container, it is considered polluted (Jutho, in Nepali) for others. Don't eat off someone else's plate or offer anyone food you have taken a bite out of.
c. Major Hindu temples are usually off-limits to foreigners. Don't enter them or take pictures unless given permission to.
d. Men should not walk/trek around bare-chested. Shorts are acceptable, but long pants are better. Women are recommended to wear long skirts. Exposure of women's legs is considered offensive, so avoid wearing shorts and short skirts. See FAQ on Trekking for more detail on appropriate clothing during treks.
e. Public display of affection is downright frowned upon. Kissing, cuddling, hugging in public is absolutely discouraged.
f. Time in Nepal moves very slowly. Everything is approximate. Nothing happens on time. Be patient. Anger and impatience will rarely make things better, if not worse. Also, double and triple confirm important arrangements, Nepalese have a way of taking everything very cool.
g. Finally, be aware that the tradition of "bakshish" --a word that literally means "gift" but is euphemistically used to refer to a bribe given in order to receive special consideration-- thrives very well in Nepal. Especially when dealing with poorly compensated public officials, a couple of rupees in bakshish, offered in a quiet and discreet manner can lubricate things up surprisingly.