As a person who has been to ground zero, just 2.5 months after the attacks, I can say the energy I felt there was very strong and something I will always carry with me. To say that anyone could stand there and simply think "Yup, bad stuff happened here" is really minimizing it (and I don't mean to offend).
I can still smell ground zero, and although I am not a chemist I am 100% convinced it was the smell of decomposition. I agree that is not positive energy but when you can see, smell and touch something it brings you to a deeper understanding of it, IMO. And, along with that horrific smell and twisted buildings, there were notes....notes of love from all over the world. There were teddy bears, there were candles, there were letters and pictures from people who lost their loved ones. It was moving, it was very deep and tragic.
I guess my point is that Ground Zero has a very very strong energy, and although it may not all be positive it is something I am glad I did because it brings me closer to those who were lost and brings a deeper understanding of our world ( good or bad).
I hope that makes sense, I probably should have just sat on my hands because ground zero and 9/11 is a very passionate issue for me.