Houston is an interesting case study. While, on the surface, it appears to mirror the other sun belt cities it is slightly different story once you analyze how things are actually getting done.
True there are no "zoning" ordinances per se but that doesn't mean there are things not directing planning in a certain direction. Let me explain the 3 major factors than CAN influence planning in Houston.
1) Private Interests.
George Hermann died a wealthy bachelor and left land to the city. He also left a stipulation on what the land was to become. He asked for some to become a park (Hermann Park) and for some to be set aside for a future city hospital. This was the site of the first hospital in the Medical Center and even without formal 'zoning' it somehow turned into one of the largest medical centers in the world. Future developers and city planners kept up with the "intent".
Rice University, The Museum District, The University of Houston, Memorial Park, St. Thomas, etc. were also created by private interests and they morphed into what they are today.
2) By Developers.
Houston is a very pro developer city. There are very little hurdles, even though they are increasing by the day, but developers with enough clout can find a way to manage their interests via Deed Restrictions. Rive Oaks is the perfect example of this in Houston. You can't buy a lot in the middle of River Oaks and build a boutique office tower. maybe on the periphery, on "unprotected land", yes but not in the middle. The NIMBY's are upset with exactly this...developments on the border of the deed restricted neighborhood.
Also developers with big pockets can heavily influence what an area becomes...think Gerald Hines and Uptown, Frank Sharp and Sharpstown, and George Mitchell and the Woodlands.
3) Management Districts and the Mayor.
This is the game changer for Houston. Both of these entities can HEAVILY INFLUENCE development patterns. Houston has a very strong Mayor/City Council setup. We also have divided the city into "mini cities" called Management Districts. Doentown, Uptown, Midtown, EADO, Sharpstown, ets. are all different Management Districts. Here is a link to more info and a map of the Management Districts:
Each Management District has a board that tends to issues "within" their management District. they have built in ways of enticing certain type of developments or discouraging certain type of developments. It's a fluid process but things are really taking shape and it makes Houston a very interesting case Study.
One of the most active Management Districts is Downtown. Together, the Mayor/City Council and the District are working at steering the type of the development in the district. the Management District ran studies on how to encourage residential and retail developments and they turned to the city for help. The city responded with many solutions...One was Discovery green which was a hybrid of #1 and #3. The City purchased the land and turned to private interests to build and manage the park. On the Residential situation, the City Council under the Mayor approved a residential incentive (something like $12,000 PER unit) for developers who build residential in a defined part of downtown. This has bee Highly Successful with a half dozen-dozen large residential projects currently under construction or are set to rise in the near future.