New York has some of the greatest buildings in the world, but it is universally regarded as the most difficult place to build. Not only are their severe land constraints, but the zoning process is extremely restrictive. Houston has no zoning and virtually unlimited land. It is universally regarded as one of the ugliest cities.
First, any other American city loses in a comparison with NYC. But for a downtown which didn't see its first skyscraper until the 70's, Houston has an I.M. Pei, two by Phillip Johnson, and a couple from SOM.
Second, are you comparing Manhattan itself with all of Houston? Sure, cut out the outer boroughs, plus the fact that most of the port facilities are now in NJ, and of course that's even more lopsided.
Now, how many of those greatest buildings in Manhattan (and they are great, absolutely) pre-date the huge re-vamp of NYC's zoning laws in the early 60's? Rockefeller Ctr, Chrysler Bldg, the NYPL, Empire State Bldg, Pan Am Bldg (yeah, Houston doesn't have a Gropius building), Lever House, Seagram Bldg, the Guggenheim, the Post Office, the Met, the UN HQ if you want to include that.... What great NYC architecture post-dates the more restrictive zoning laws? The WTC - is that the exception that proves the rule?
Anyway, all of this is to say it seems to me that Manhattan's collection of great buildings comes mostly from a stunning concentration of wealth.
Otherwise, what David and Andrew said.
Btw, I think Houston does have one zoning law: Every strip mall must have a nail salon.