Around April-May 2013, Tesla changed the design of the vents on the Model S in the front windshield to improve defrosting. The older design sends the air through a perforated screen, while the newer design has larger directed vents molded in plastic. Tesla will change the old design to the new one on request for free.
The design was changed again with the April-2016 refresh, where Tesla reverted to the clean mesh look, but the vents under the mesh have improved and work well.
I have the older design in my first Model S, but have only used defrost a couple of times in a year and find it works fine in California. I prefer the cleaner look of the old design than the modified visible vents, which reflect slightly on the windshield.
If you have the old design on a pre-refresh Model S and are not getting adequate defrost or the windshield is fogging up, you should consider getting the vent retrofit as users confirm it does work better.