I would rank nanotech as fairly low on my list of concerns, because cells are fairly close to optimal replicators in the present environment. (IE, I don't buy the grey-goo stories: the worst that I find plausible is a nanobot plague, and normal biological weapons would be easier to make.)

I'm not sure whether this response is "within the spirit of the question" - but the primary actual problem with nanotechnology - if it is somehow magically delivered into our hands without superintelligence - is that it then massively facilitates the construction of intelligent machines and robots - by providing a high speed, low cost, efficient computing substrate - along with sensors and actuators for it to use.