It appears that hurricane frequency and intensity is controlled more by some long-term cycle that we don't understand than by global warming. We had a bunch of hurricanes in the 50s, then a long quiet cycle while people built a bunch of stuff on the coastline, followed by another active cycle starting around 1995.
My personal (and slightly educated) opinion on why Katrina developed in such a nasty way follows:
We started out with a very active hurricane season this year. Then, we went through a substantial period -- about a month -- where conditions were temporarily unfavorable for hurricane formation. We had Harvey and Irene in the Atlantic, neither of which were any great shakes, followed by Jose which blew up and immediately down as it hit the Yucatan. That wasn't much activity for August.
If Katrina had followed a normal path, it would have crossed Florida from east to west and curved north, pretty much following the coast. It never would have gotten very strong and would have dumped a lot of rain and done a little bit of damage.
Instead, Katrina was blocked and veered south across the Everglades. It didn't lose much strength crossing the wetlands and it got into the Gulf of Mexico, which was very warm because there hadn't been any tropical cyclones churning it for quite some time. The storm moved really slowly, still blocked by the atmospheric phenomenon that had forced it south, giving it plenty of time to intensify.
Once it got around the blocking event, it could recurve normally, but now it was a Category 5 storm aimed at New Orleans and the central Gulf Coast.
So we got a catastrophic storm instead of a couple of smaller ones.
Not Wrath of God. Not global warming. Just some bad luck capped off by a big blocking area of what I assume was high pressure. (But I'm an amateur meteorologist. We don't get the good maps.)