All right, here's the chemist in me coming out
Bleach contains chlorine which is a fairly caustic element when it's in liquid or gas form. It's fumes can irritate the lungs and respiratory tract. It's corrosive so can damage/irritate the skin, eyes & other membranes. Also, when used in washing machines, the chlorine can be released into the air, basically as an air pollutant. And if mixed w/ammonia becomes a dangerous gas (this is very very very BAD).
Here's an interesting website that talks about the dangers of common household cleaning products including bleach:
http://www.mamashealth.com/doc/cleanprod.asp