From what Ive read and seen, there is a cyclical pattern of warming and cooling of the oceans (el nino anyone) Seems we are in the time of a "warm" trend. I think the last warm trend was about 40 years ago when Camille and Hazel(i think it was Hazel, which made it all the way to Toronto) happened. It might be hard to say if Global Warming is as big of a trend as we are lead to believe or that this "warm" trend is part of it. (Havent heard any updates about the holes over the antarctic lately)
As for all the damage, I think it was
1. The way the city was designed
2. The relative lack of experience the residents had with major storms (compared to say...Florida)
3. The lack of leadership and subsequent emergency relief
These people that say God is punishing us for having abortions (the hurricane resembling a fetus) or for plain old sin, I think is a stretch. If God really wanted to teach us a lesson hed send another flood. but than i think the bible says that he wouldnt do that, the next time it will be fire!