The water came up relatively slowly compared to what happened in Baton Rouge so they were able to save people as different areas flooded and it gave them time to bring in the rescue teams.The other thing it had going for was about 10,000 rednecks with bass boats who were bored and looking for a reason to drive their boat down the middle of the street. A big plus is the for the most part the water was moving very slowly. Fast moving water destroys everything in its path. Houston is really big, for all the press coverage most of the area did not get major flooding. No one I know down there had water in their house even though lot of them were stranded because the roads were flooded and I have been in contact with a couple of dozen. My parent's old house was destroyed but it was in flood plain very close to the San Jacinto river.
I saw some criticism that they were not evacuated but I think more would have died if they had evacuated. Besides where would you send 6 million people? New Orleans is only 400,000 and that strained the areas ability to house them. The Houston area has twice the population of entire state of Louisiana. The scary reality is we really don't have the ability to evacuate major cities in any kind of reasonable timeframe.