I think people who didn't play this upon its inception may not truly understand the beauty of COD4, especially if they came to the series aftewards.

Call of Duty up until 4 had always been a fun series, but never was exactly the best game in the FPS genre. Want to play an engaging experience with good maps? I'd wager for Medal of Honor. Want to experience combat on a larger scale? Play battlefield. It was here that Call of Duty finally broke away from the mold and became the massive franchise that it is today.

The campaign was very good for an FPS to start, I think just about everyone will remember "All Ghillied Up" for sometime where people got to channel their Solid Snake for bit. The real icing on the cake though was the multiplayer which had varied weapons, perks, and best in class maps that made experimenting enjoyable and playing hours upon hours on end joyful. COD4 hit every mark when it came to "FUN" and had the best ideas in the series before the series would devolve into multiplayer degeneracy in MW2 (which was still fun, yet very broken.)