The singleplayer is all right, it has a strange storyline. The multiplayer isn't that fun though. It is very flawed.
As for multiplayer, it is very flawed. I'll just go over some examples:
Spawning
This doesn't happen that often, but it still happens. Sometimes while playing, I'll spawn right around the corner of an enemy, and then die within seconds of spawning. Also, I have been killed by the same air strike twice, many times. I'm not saying that Halo 3 is a better game, but they have a better spawning system. You know the little blue spawn points you can make in forge? Those are in ranked and social games. You don't just spawn at one of them randomly. Bungie has a point system for each spawning location. For every teammate near it gives it +1, for every enemy it gets -1. This makes sure you won't spawn near enemies. Infinity Ward should do something similar.
Too easy to kill
Someone may think "Why complain if it's easy?" If it's easy to kill, then it's easy to die. Infinity Ward changed something, because Call of Duty 4 was really fun, and that's a big part in why Modern Warfare 2 had so much hype. Call of Duty 4 is easier because it's harder, if that makes sense. MW2 let's anybody play. It's at the point where a 6-year-old can be good. If you have a 6-year-old play MW2, and then play CoD4, they will most likely do better at MW2. I even tested this and it's true. 6 year olds can play, which makes it annoying because everyone is good.
Nuke
This shouldn't even be a feature. The nuke influences camping, the use of overpowered kill streaks (I'll get on that later), and soloing everything. If you ever happen to get a nuke, you probably received the "Most timed spent in one place" accolade aswell. You also probably got most of your kills from the overpowered AC130 or Chopper Gunner. Lastly, nobody works as a team. Everyone is too busy trying to get a nuke or something like that. Working as a team usually results in a win, or an epic fail with major lulz moment. Working as a team would be fun, but everyone just wants to run 'n gun or camp by themselves, and then boast about how they have a 2.5 K/D ratio.
Kill Streaks
Here is another case where CoD4 was better. It was fun when everyone had UAV, air strike, and attack helicopter. Since the game was hard, it was rewarding to get an attack helicopter, even if somebody shot it down. Now, nobody uses UAV or Conter-UAV because it only helps the team, not get them more kills. Also, very few people use anti-air equipment, most likely because they have One Man Army for infinite ammo or the Model 1887′s. This causes the air support to stay up for the full length of time. So add overpowered air support with a minute or two, and you receive about 5-10 deaths. You may say "then use cold-blooded," and I'll go over that in the next paragraph. Also, it doesn't make sense to reward the team that is doing good, just so they can do better. If a team is winning 750-200 then why reward them with air support? It only makes sense to reward the losing team to keep the game challenging.
Stopping Power
Face it, if you don't use stopping power, most of the time you will either get out-gunned or quickly run out of ammo. Playing on hardcore doesn't really fix this because there aren't a lot of objective type games to play. Search and Destroy, well, I suck at search. Headquarters is a joke. This new "headquarters pro" didn't stop the 250-0 ending score for every game of headquarters. I enjoy playing demolition, but sadly there is no hardcore option for it.