The 212X is the upgraded version that hasn't reached proliferation yet. Fan aside, the difference lies with the aluminum fins. They have X-shaped perforations around the heatpipes. These perforations have a two-fold purpose: the first is to help direct some of the airflow around behind the heatpipes to stop "heat pockets" from forming, the second is to allow for better a better type of airflow.
In a previous generation of aftermarket coolers, Enermax included such features on their ETS-T40 heatsink, which was a direct competitor to Cooler Master's Hyper 212 Plus (along with the Xigmatek Gaia, I believe). Other companies are starting to adopt the same ideas.