The opportunities in some ways are almost unlimited, the ability to craft nano-particles that could, for example, scrub the plaque from the inside of arteries or perform other functions within the body, etc. The applications are obviously limited by the level of complexity that can be reached at a given size, but nano particles are already in use in all kinds of applications including physics, engineering, medicine, etc.
The idea of the dangers of it is a difficult concept to get one's head around. The problem with technology is that usually we rush into the new technology because it is sexy, often profitable, etc., without considering the implications of it. If we look at the previous application within human arteries, does it then lead to people being irresponsible with their health because this new technology can absolve them of the consequences of their poor decisions?
Anytime you replace a current technology or practice with a new one, you are getting rid of something and we rarely stop to consider all the implications of it, ethical or otherwise, and so the dangers usually only appear after the fact.