In the end, Modern Warfare 2 is mostly more of the same in this highly overrated and oftentimes incoherent series.
1) In the main campaign, the storyline is not engaging, and even the immense set pieces aren't able to salvage the mediocre single player. Simply put, this game lazily explains the storyline with short briefings between each mission. Your companions are mostly well-animated, but they lack distinctive personalities. It's just a series of set pieces without any real explanations or substance. It's like Infinity Ward desperately wanted everything to look "cool", coherent storyline and likable characters be damned. As far as the new Russian adversary is concerned, he's OK, but Modern Warfare's villains never reach the intriguing heights of a character like Andrew Ryan. He's just...there.
2) Your A.I. companions are decent as far as intelligence is concerned, but Modern Warfare 2 lacks something that would have been a significant step forward: the ability to control members of your squad by issuing commands, setting positions for them to take up, etc. Yes, their intelligence is serviceable, but it would be nice to have the option to customize how they approach a situation. Having a friendly stupidly block your path or run straight into gunfire during a chaotic firefight will infuriate you to no end.
3) You should have an interest in multiplayer if you decide to purchase Modern Warfare 2, because the single player is pathetically short, and you don't have many reasons to re-play the same missions (if you do, I would advise turning the volume down, since the dialogue tends to rely on cliches). That's a shame, because Infinity Ward sets up a pretty promising foundation that never fully reaches its potential.
Now, the visuals in Modern Warfare 2 are simply stunning, which is why I decided to raise the score. However, it's a shame that most of the levels are linear, and you don't have too many options for handling a situation. Being able to control members of your squad would have definitely made progressing these levels much more tactical and intriguing. As it is, Modern Warfare 2's single player favors "cool" moments and pretty packaging over a gripping storyline that makes sense.
MW1 was mostly grounded in reality, but some of the events in this game are laughably implausible. I must admit to being disappointed by Infinity Ward.
Multiplayer is good, assuming that you are able to avoid the moronic children and elitists that seemingly think that their online reputation is comparable to being a recipient of the Nobel Prize.
Special Ops is the most intriguing mission, but it fails to live up to its promise. Why can't you invite more than one additional player? Where is the customization? Many of the levels feel mostly recycled, and the mission objectives just aren't that creative, save for a pretty fun match in which one player controls an aircraft while the other player attempts to complete some critical objectives on the ground.
In the end, Modern Warfare 2 needed a stronger storyline, more memorable characters, and less of an emphasis on "COOL!" scenes that lack any connection to the rest of the storyline. If you thought that Modern Warfare 1 was more hype than delivery, don't expect to become a convert after MW2: it simply makes a few tweaks without addressing many disappointments, namely how painfully short the otherwise promising campaign is.
Multiplayer is the real attraction here, but the limitations on inviting other players is unfortunate. They also threw in a third-person perspective for some online matches, but it feels as awkward as controlling a third-person character in Fallout 3. You can barely cover, and shooting feels too difficult. There's no use in introducing another perspective if you don't alter the surrounding environment/options to make it feel natural, as opposed to forced and unnecessary.