bheid, the biggest problem is the rate that they grow. 20% of broilers have chronic leg pain/deformities/etc. Many can't walk or reach the water and die of thirst. That's a pretty awful and inhumane way to die. As they grow their hearts and lungs give out, another ****** way to die. The chicken industry accepts the high mortality rate because it's more profitable to grow big chickens fast and still lose a **** ton to thirst and lung failure.
Also, there's usually 10,000 packed into one grow house. So the air is pretty horrible to breathe. Not sure how much that really affects chickens, though.