It primarily removed Punkbuster support (which was already obsolete) and allegedly fixed a few Remote Code Execution (RCE) exploits.
It allows for features like higher player counts (up to 64), better hit detection, and the ability to download custom maps and mods automatically when joining a server. cod4 18 patch
Since Activision's master servers are often down or broken, CoD4X provides its own master server list so you can actually find games. It primarily removed Punkbuster support (which was already