m0tch tries to review this game properly. (Modern Warfare 3)

Alright, let's look at this as a game on it's own, and not as part of a series. Call of Duty: Modern Warfare 3's single player campaign plays like a movie, with little emphasis on exploration or tackling objectives in your own fashion. Behind all of the explosions and mayhem is a glorified set of corridors in which you do exactly as you're told, or else you cannot progress. Shoot the helicopter with this conveniently placed RPG, jump here, bomb these red squares here, follow this guy, and so forth. One part of the game attempted to make me feel something, but it didn't. Anyone else who played through the campaign knows what I'm talking about (it involves a van).
The gunplay in the singleplayer seems to be a lot like whack a mole, with you trying to shoot the bad guys before they pop into cover, without much variation. 
For a lot of people, the main meat of the game is in it's multiplayer. This mode contains your standard modes of play such as deathmatch, team deathmatch, capture the flag, and so on. You get perks, which are like attributes that improve your abilities in certain areas, like sleight of hand which makes you reload faster. You also get certain "streaks" which are rewards for doing well. An example would be killing three people in a row and receiving a "UAV", and this UAV grants you the ability to check a minimap for the location of your foes. You can also get "streaks" for doing various teamwork related actions. 
A third mode is the "Spec Ops" mode where you can attempt a few challenges either alone or with one friend. There is survival mode within this mode, where you fight wave after wave of enemies for as long as you can, while being able to buy upgrades and other weapons between and during waves to heighten your chances of survival. It's basically like the game Killing Floor with some more running. The other chunk of spec ops is a bunch of varied challenges, like "kill these targets within a certain time to receive stars" or "collect these samples". The stars you receive allow you to unlock more challenges, and playing the challenges on higher difficulties rewards you with even more stars. 
Now, I can't give this game two ratings, but I'd like to. If this is your first time playing a game in the Call of Duty series, I'd give it an 8. If you have played games in this series before, 6, because it is very similar to the earlier ones.

What the rating is out of is completely up to you, as usual.