Are you serious? This is a Call of Duty Game? Ranks, Challenges, Amazing online play....impossible.
The single player is great, but incredibly short. A person new to the call of duty series, would be able to beat call of duty 4 in a matter of 5 hours or less. I myself beat it in 3 hours, but those 3 hours were amazing! There are amazing cutscenes and interactiveness. There are amazing missions where you are up in a helicopter shooting rockets down at huge armies, and it was too much fun. The Single player I would give a 8.5.
The Multi-player now has Ranks/lvls 1-55 and it gives a point to the online play. It keeps track of your kills, deaths, time played, assists, headshots, total score, total xp and so on. There are challenges that you can complete for XP towards your rank, and as you complete the weapon challenges, you unlock add on parts for you weapons such as silencers, red dot sights, an acog scope and a few more. Along with that completing other weapon challenges will give you different camo schemes for your weapons as well. As you rank up you unlock new weapons, new challenges, and more. The game is almost never ending, and incredibly interactive, and theres a point to everything you do. The Multi-Player I would give a 10.