I also believe that nuclear power is a good idea. But, if pressed into a devil's advocate situation, I would say:
1) The high initial start-up costs. 2) The radioactivity of the waste products of fission. 3) The potential of an ecological accident resulting in radioactive material being released into the area. 4) The potential for terrorist attack on a nuclear facility, also releasing radioactive material. 5) The lowering of property values around the area of the nuclear plant can hurt the area's economy.