It may sound obvious to anyone who knows me, but “hazardous, tricky clean-up of nuclear sites”:http://www.newscientist.com/news/news.jsp?id=ns99996199 is for me the single compelling reason to avoid nuclear power as a major energy source. I don’t worry nearly so much about Chernobyl-style accidents as I do about subtle, continuous contamination of an ecosystem due to nuclear byproducts.
Note that this does not apply to cases where nuclear power provides a clear advantage, especially when the contamination argument is less prominent. Specifically, interplanetary spacecraft are a good use of nuclear power, because the need for a lightweight power source is so prominent. I tend to balk at the idea of using nuclear engines for propulsion — at least in the long term — but using a nuclear source for electric power makes perfect sense.
Enough of that, though; I’m boring myself.