In the first Call of Duty I felt like I was a war, UO took it to a a bigger scale with deployable weapons and vehicles. The second game was a watered down version of the first because vehicles and deployable machineguns along with the healthbar were removed. Finest Hour brought vehicles back, but it was a console exclusive. Call of Duty 4 brought 64 multiplayer maps aswell as unlocks and a the ability to call in airstrikes in multiplayer. Every game after that had only had a few minor changes that didn't even make the games worth buying.