Here's a cut-away view that shows how New Orleans can easily turn into a lake.
Earlier today this page said the following, but it's since been deleted. I don't know if that means they no longer believe it, or if they thought it was a little too wrath-of-god:
Most of the area will be uninhabitable for weeks...perhaps longer. At least one half of well constructed homes will have roof and wall failure. all gabled roofs will fail...leaving those homes severely damaged or destroyed.
The majority of industrial buildings will become non functional. Partial to complete wall and roof failure is expected. All wood framed low rising apartment buildings will be destroyed. Concrete block low rise apartments will sustain major damage...including some wall and roof failure.
High rise office and apartment buildings will sway dangerously...a few to the point of total collapse. All windows will blow out.
Airborne debris will be widespread...and may include heavy items such as household appliances and even light vehicles. Sport utility vehicles and light trucks will be moved. The blown debris will create additional destruction. persons...pets...and livestock exposed to the winds will face certain death if struck.
Power outages will last for weeks...as most power poles will be down and transformers destroyed. water shortages will make human suffering incredible by modern standards.
The vast majority of native trees will be snapped or uprooted. Only the heartiest will remain standing...but be totally defoliated. Few crops will remain. Livestock left exposed to the winds will be killed.
And what lives in the water in that part of the country? Alligators and poisonous snakes? Oh, that the basin also has above-ground graves, chemical plants, and (like most cities) sewage treatment plants. [...] The live feed reporters also just said, "Oh, and fire ants." They'll swarm floating things to save themselves, then people out there wading grab the floating things, and... (the reporter actually shuddered a little).
A couple of older articles about how screwed New Orleans is: What if Hurricane Ivan Had Not Missed New Orleans? And Thinking Big About Hurricanes.
Rescue teams would have to don special breathing equipment to protect themselves from floodwaters contaminated with chemicals and toxins released from commercial sources within the city and the petrochemical plants that dot the river's edge. Additionally, tank cars carrying hazardous materials, which constantly pass through the city, would likely be damaged, leaking their contents into the floodwater and adding to the "brew." The floodwater could become so polluted that the Environmental Protection Agency might consider it to be hazardous waste and prohibit it from being pumped out of the leveed areas into the lake and marshes until treated.
Lots of info at Wikipedia.