Spoilers, but you've already played this so it doesn't really matter.
The campaign is just so poorly put together. You play five different characters over the course of the single-player campaign and three of those characters die with one being gravely wounded. In the first Modern Warfare game, you played three characters and one of them died. When they died it was shocking and quite effective. When the three characters die in MWF2, it feels forced and really unnecessary. This is especially true of the astronaut portion where you float in space for a second and then get blown up for no real reason. I don't think they even name the poor guy. Even when the game is picking up steam, then completely break the tension when they switch to another character. For instance, there is a scene where you are an American soldier defending Washington DC from the Russians (because we are living in 1984 for some reason.) The helicopter you are in is shot down and you are being surrounded by the enemy. Ammo is running low. It looks like everything is lost and then...it switches to a different character. Granted, it shows how what happens afterwards came to be, but did it really need to be shown. It would have been way more effective if it had happened and then the player found out why. In fact, when the why is shown, it still doesn't make sense. I don't want to give it away despite the spoiler warning, but it all hinges on a character who is always portrayed as good allowing something bad to happen without anyone giving any real reason why. This is another common with the narrative; people do things that make absolutely no sense and then a bunch of people die. It's stupid and frustrating. I expect better from Lance Henrikson, Captain Anderson and the guy who played the sniper in Saving Private Ryan (they all lent their voices and, with the exception of Captain Anderson AKA Keith David I didn't realize it.)
Strangely enough, the most interesting, fun and well-made part of the game is simultaneously the least realistic the invasion of America. I'm tired of the Russians as bad guys as much as anyone who doesn't masturbate to Red Dawn but the environments are so interesting, the missions are varied and, most importantly, the story is actually cohesive. While America is invaded, you play as one character, in one squad and you stay in an area that is both familiar and varied. It's not realistic, but games don't have to be and, often, shouldn't be. In a space where you can create anything, why would you create something exactly like the world around you?
So the single-player campaign with it's pointless deaths and poorly constructed narrative is fairly stupid. It doesn't matter, though, because people bought this game for its multiplayer. The single-player campaign is little more than a glorified tutorial meant to teach newbs the basic mechanics so as to avoid a Counter Strike-sized learning curve. Multiplayer used to be huge when I was living in a house with five other guys, but playing it again made me realize that it's really nothing special. Maybe its not fair looking back on this game after everyone who owns a game studio decided to steal their format, but especially after Call of Duty:Black Ops introduced wager matches, MWF2's multiplayer is lame. The killstreaks are still kind of cool and all, but at this point the only people left playing are the hardcore douchebags who couldn't move on when the new games came out. Even so, the matches are often glitchy with connection issues often cutting games short. The gameplay itself isn't particularly varied, with the regular deathmatch, capture the flag and territory holding matches that have been around since online games themselves. The guns are fun to shoot and all, but it's nothing special.
I realized that the reason my friends and I enjoyed those matches was because we were out on our own for the first time and could stay up all night with two walls of TV's constantly playing and shouting with each other. In the beginning, because Infinity Ward decided that beta testing was for chumps, the games were so glitchy and easily modded that the matches were all-out insanity matches. Hosts would pile thirty people only the smallest map so that every time the player spawned, eight people were below them. With no kill cap, a player could get sixty kills in a match. It was ridiculous, but it was fun. This is what made the game good and fun to play; friends and insanity. Once they lost those, we were left with uninspired multiplayer with a few bells and whistles attached and a single player campaign with a few good moments that were snuffed out by a stupid narrative structure, ridiculous plot twists and ethical problems. I forgot to mention the implied torture that one of the supposedly good characters engages in that is never mentioned and has no repercussions. So there's that. Overall, it's a really misguided game that made a lot of money because it is easy to play with your friends. I'm sure Modern Warfare 3 will sell plenty of copies, but I'll be too busy playing Skyrim to care.