Most likely the best First person shooter ever
Let's start with the single player. In my opinion, this is on of the best single player FPS campaigns of all time. Some of these battles are just plain EPIC, like Red Square or Berlin to name a few. The gameplay is flawless. The normal WASD movements, mouse to aim, left click to shoot, and right click to aim down the sight. It is a pretty good length for a FPS campaign... at least if I remember correctly. I lost track of time while playing.
You play through four campaigns, one as an American, one as a British soldier, one as a Russian, and one with one mission from each soldier. Every campaign you fight against the Nazis, but the situations are different enough to keep you coming back for more. Also, each soldier has different weapons, which is a very nice change of pace. The enemies aren't stupid either. They won't stand in front of you like an idiot, they'll shoot. If they're close enough, they'll use melee attacks. These guys are not dumb, and should not be messed with, especially on Veteran.
The graphics are very nice for a game from 2003 as well. People do not look like LEGOs, they look normal. If you throw a grenade into a room and run in afterwards, there will be black marks from the explosion. Not exactly Frostbite engine material, but for a game as old as Call of Duty, it's damn good.
Now for multiplayer, it's a little small (16 players, 8 on each side), but it still has a nice community going. Sure, they're really good now, but it's still fun. There are your normal online modes, but they do it well. Call of Duty's strong point is single-player (very off for a FPS) but the multiplayer adds plenty of extra playing time.
This is my favorite first person shooter of all time. You can argue all you want for Halo, Doom, and Wolfenstein, but this is just pure gold. I don't care if it takes place in the worn out time of WWII, it is still absolutely incredible. It blows every other first person shooter I've played out of the water. Only game I have ever played that actually deserves a 10.