I want to start by saying that i really think that im the biggest dog lover on the planet. I always have to pet every dog I see. Pitbulls included (though I have bit of trepidation when i pet them). Most pitbulls will be great dogs just like every breed of dog. They are almost all great.
The big difference is that if a dog attacks, at that point the breed makes a huge difference. Labs are pretty big dogs. Generally bigger than pitbulls. Yet if a lab attacks me im not in any real trouble. I will probably need a few stitches and have some bruising but will be ok. If a pitbull attacks me then im in serious danger. Probably still wont die but a decent chance that I will be maimed and have life long injuries.
Plus, everyone has a story of a pitbull attack. For myself, a pitbull jumped our back fence and killed our kitten (I have had 5 cats in my life and that one was my favorite

) another pitbull attacked my dog (a healer/cattle dog mix) and left him pretty bloodied up (he had no chance in hell of standing a chance in the fight) while we were on a walk. It came flying down the street after having got out of the house and attacked my dog with no warning.
A different pitbull attacked my dog on a camping trip and left him bloodied up once again. Lucky in both cases the owners came and got hold of their dog or my dog would be dead.
They are just different enough to be way more dangerous. And again, im a HUGE dog lover and I love pitbulls too but with so many amazing breeds out there available I cant understand why people would choose a pitbull.