The levees were designed at a lull in the hurricane cycle so the risk of a Cat 4 or 5 hurricane is probably higher than was believed (about once in 200 years) when they were designed. Given that the levees were never designed to withstand anything above Cat 3, it is unconscionable that Nagin and Blanco didn't order a mandatory evacuation 48 hours or more ahead of the projected landfall. Considering the Louisiana state evacuation plans called for the Governor to commandeer all state owned transports, there were over 2,000 school and transit buses available in the state available to evacuate people who had no cars from New Orleans.
You are right about the mandatory evacuation....it should have been done.
But wait....we can always blame Bush! /sarcasm