Forwarding ports is a useful trick in gaming because it can really help to make your network connection most stable and intermittently even faster. When you are playing Left 4 Dead 2 you might need to forward some ports in your router. They generally focus on conserving resources such as ammo and health, and sometimes employ elements of stealth.
Many are horror based, supernatural, or post-apocalyptic. Some have waves of enemies that get harder with each wave.