Call of Duty 2 takes all the elements that made the first Call of Duty great and makes them even better. There are a few more weapons to utilize, more locations to explore (North Africa, Moscow, Point du Hoc on D-Day) that really make Call of Duty 2 shine.
Call of Duty 2 also features a much more involved series of tank battles that require you to shoot more than enemy tanks. The first Call of Duty allowed for a flexibility in completing mission objectives that few other games offered. Call of Duty 2 takes that even further, allowing you to complete several objectives in any order you wish.
Call of Duty 2 also introduces the feature of smoke grenades. These allow you to conceal your advance from snipers, machine gunners or tanks. This adds a whole new tactical dimension to multiplayer battles.
The most notable feature of Call of Duty 2 is the elimination of the health bar. It has been replaced instead with a system of recovery. If you get hit, your vision will blur with red. You must extricate yourself from combat to avoid being killed. After a few seconds, the red subsides and you are ready to go back into action again. This feature has now become commonplace on nearly all first-person shooters.