0) More fan noise
1) EM Radiation, wont kill you, but will probably mess up radio/TV/wifi reception.
2) No airflow over the motherboard caps can cause them to overheat and catch fire.
3) Electrocution from small fingers/paws touching it.
4) Short circuit from dropping crap in it and the bottom touching something metal.
5) For god sake don't put it on anything flammable you could burn down your house down.
davincij15's cardboard server cabinet gives me nightmares!
http://www.youtube.com/watch?v=8lkrGimFimk#t=0m56s