Nepal is not the West - wonderfully and magically not! Many things here will delight and amaze you, and some things will shock or surprise you. That's part of the fun of travel.
We believe that the people who try to see the world through Nepali eyes while they are here have the richest experiences. If you want to do that, think about a few things:
There is a strong social hierarchy in Nepali society, based largely on age. Family is extremely important, and familial-type relationships extend outside the family. Nepalis refer to each other consistently with terms like "older brother" and "sister-in-law", even when they are not related. There are many degrees of politeness and formality in the Nepali language, and people vary their speech depending on the relative age and social standing of the person they are addressing. The best that travelers can do in this complex context is to be uniformly polite to all Nepalis. It is appropriate to be slightly deferential to those older than yourself, to someone who has rank or a title, and to someone to whom others are being especially polite.
Religion is a powerful part of everyday life (and indeed of Self) for most Nepalis. Nepal is unique in the world for the way Buddhism and Hinduism are fused, even though each individual is either Hindu or Buddhist. You will see shrines and temples in an amazing variety of forms and sizes, daubed with colored powder and offered fruits, flowers, rice, sweets, and money in worship.
You should get up very early one morning and walk through the old city to see people making their offerings. Watch the way that religion is interwoven with everything else, from the commuters touching a shrine hurriedly as they head to work to the people bathing and doing laundry at the temple spout and then hanging the clothes to dry on the temple itself.
Be sensitive to a subtlety of language. Nepalis are less direct than Americans when speaking in English, and most Nepalis will be deferential in their approach to you. What sounds like a question may really be a suggestion, and a suggestion actually something more. Some Nepalis will be uncomfortable saying "no" or otherwise disagreeing with you, so you might prefer to phrase yes or no questions as a choice - "Has this water been boiled or not?" That formulation of the question is closer to the way one would ask in the Nepali language, and is therefor also more likely to be understood correctly. When possible, answer questions without using a direct "no". "We would prefer to stay in the hotel this afternoon" is a very polite way to say no to "Would you like to go for more sightseeing now?"
Try to observe a few basic social norms which are different from those in the West, and be aware that when people in Nepal do things which seem odd to you, they are probably not odd here.
Food should be touched and eaten only with your right hand, unless it is impractical to do so. This is particularly important when taking something from a common plate or bowl.
The head is considered to be the "cleanest" part of the body, and the feet the most "impure." You should avoid touching people on the head or with your feet, and you should avoid pointing the soles of your feet directly at anyone.
In many religious sites and people's homes, shoes are not worn. When entering any building or room, look around. A pile of shoes is a sure sign you should take your shoes off too. As with most social norms, take your cues from what the Nepalis do.
Stepping over people sitting or laying on the ground is ill-omened and impolite. Avoid walking between people and a fire or stove if possible.
Dress modestly. In Kathmandu and highly traveled areas, people are used to western eccentricities such as shorts on men and pants on women, but in much of the country those are off-putting to people. Short skirts are definitely out. Men dressed in long pants and a shirt, and women in a long skirt or dress with their shoulders covered get the best reception from the local people while on trek.
Suspend your "Why?", or at least the ready verbalization of the question. It is very strange to Nepalis that when confronted with a fact or situation, foreigners often ask, "Why?" Nepalis do not do that. Usually we simply want to know; but even when we mean no criticism by the question, it's often taken that way. Accept and be patient. That's the Nepali (indeed, the eastern) way.
Consider that the person to whom you are speaking may not know "why" (he or she almost certainly didn't ask), and to admit they don't know would be a loss of face. If you need to know "why", consider forming the question like "It there a problem with X" (take a guess), "or something else?" Interpret a vague answer as "I don't know."
Remember that exact directions and times, precise dates on things, and consistent stories about gods and kings are fixations of our culture, not Nepal's. They don't particularly care how old a temple is, for example, just about the god within. And if you ask three people outside that same temple what god they are worshipping there, you might get three different answers, all correct.
That sometimes a place is known by different names, or that Buddhists and Hindus may worship each others' gods and saints, or that a festival can have multiple meanings to its celebrants is just part of the rich fabric of this society. If you're the type of person who likes to "get things straight," we will be happy to refer you to some excellent books which attempt to do just that.
Expect to bargain for purchases, and try to enjoy the process. Restaurants, bookstores, and a few shops have marked, fixed prices; but anytime you need to ask a price, bargaining is expected. It's a social interaction, not an adversarial thing. Relax, participate, and in the end pay what you think is a fair price or walk away from the deal.
Most of all, while you are a guest of the wonderful Nepali people, try to see and experience the world as they do. You will find the experience rewarding.