When I thought of dangers in Nepal, I thought of Maoist or Tigers. It turns out riding the buses is much more dangerous than both combined.
Fatal accidents are an everyday occurrence as we witnessed on our trip to Chitwan National Park. In a distance of about 100 miles, we saw one bus being pulled out of a 15 foot ditch (lucky since this is one of only a few areas where the drop offs are small) and a natural gas tanker overturned. On the return trip, we saw a different overturned bus and a bus with a motorcyle under its wheels.
Lonely Planet (the guide book) has a blurb about the 10 accidents that killed 200 people during the ten days of their research on the book. It seems that the rule of law and common sense are totally absent in this part of the culture.