Mass Effect 2 is something special. There are few games that have ambitiously attempted to really bridge the gap between gameplay and narrative - and Mass Effect 2 seems to actually succeed.

Mass Effect 2 picks up shortly after the end of the original Mass Effect. Commander Shepard, the series protagonist, defeated the vanguard of an galactic annihilation by an artificial race called the 'Reapers' in the nick of time. After this remarkable feat, Shepard and the Normandy, the players' spaceship are sent to clean up some loose ends.
In a suprise attack by a giant unidentified space-cruiser, the Normandy is outgunned and destroyed in seconds, and Shepard is killed while saving his crew. Cerberus, an extremist pro-human organisation 'rebuilds' Shepard to defend against the threat of the Reapers; the political elite discards the Reaper threat as a problem solved - Shepard and Cerberus know better. Cooperating with the group, Shepard sets out to assemble a team to enter through the elusive Omega-4 relay - a warpgate no ship has never returned from.
I can summarize what I feel about this game in a single example. The moment the new Normandy, secretly rebuilt by Cerberus, was revealed to me I smiled on the exact same moment Shepard did. As the pilot that I died saving in the destruction of the original Normandy stood next to me, he asked me "It's good to be home, huh, Commander?". And then, as the Normandy took off, I involuntary had goosebumps. Then I realized, this is what emotional connection feels like.
The gameplay consists out of third-person action gameplay, roleplaying gameplay and a extremely powerful conversation system. Surprisingly, the elements blend perfectly into something more than the sum of parts. It has the best elements molded together into a tasty blend of everything good. A bit like the Powerpuff Girls, I suppose, but better.
The combat is fluid. Taking the variety of the original games' combat system, it has been distilled and improved to something more efficient - something more natural. Now, you can order one of your squad-members to take cover, while ordering a second to lift up an enemy with their biotic powers, while you decide to recharge your barriers at once. You can use pistols, rifles, rocket-launchers and even tactical nukes to destroy the heaviest of armored foes, or you can lift, throw and blast enemies through the air. Since you have an emotional bonding to your team, their well-being feels important, even if they can be revived easily. Which brings us to the characters.
The strongest part in Mass Effect 2 are the characters. During the game, you will interact with many diverse characters - almost every single one with their own story, background and problems. Depending on how you interact with these beings, and how you handle the situations they present to you, you end up with different conclusions and results. This is the power of Mass Effect 2. More precisely, the power of Mass Effect 2 is the feeling that you, as much as the characters, belong in this fantastic universe under siege. The sincere belief that your choices are shaping it. Not the awesome action sequences, nor the well-told story, nor the jaw-dropping cinematics or the well-composed sound(track) can match this feeling - even though they all take part in conveying this sensation to you.
Dialogue is convincing. For a Mass Effect game it's not that surprising that the cast is impressive for the second iteration - but Bioware went all-out this time around. There is actual emotion in both the models and the audio, which soldifies the gameworld from loose characters sharing the different environment to a powerful, vivid universe filled with people that belong. It's one of the few games I've played where I've taken the time to actually exhaust every possible conversation completely before moving on, hoping there was a entry to more information or different solutions to a problem beyond those. Actually, the only other game that came close to doing that to me in the last few years was the original Mass Effect. Improving upon the original, not only are there more choices and more logical charm and intimidation systems, there is the added bonus of an interrupt system. At some points, a flashing icon will appear during dialogue and pressing the "Good" or "Evil" button will make Shepard do something heroic or bold. Not only does it add to the conversation, it also feels great to be good Bruce Willis or bad Bruce Willis.
The team is the best example of this. In the game, Shepard sets out to recruit a new team of highly specialized professionals. Scientists, vigilantes, psychopaths and warriors of every race, Shepard has to find them, recruit them and then get them to follow him. Whether they trust you, however, is a completely different story, and to get people to trust you you have to gain their loyalty through conversation, manipulation, support and sometimes - if you want to - romance.
Adding to this is that you can import your Shepard from the original Mass Effect title. Your choices from the first game cascade into the second - sometimes subtle, sometimes dramatic. Either way, whether you have helped someone out or have taken someone out, whether you've been nice or a jerk, whether you've done a certain quest or not - it all has an effect on the second installment of the series. More interestingly, your choices in this game will cascade through into Mass Effect 3, creating a personalized, individual story for each and every single player.
Of course, the game is not perfect. There are mistakes, people that run out of dialogue halfway through the game, some bugs, some illogical choices, but these are all forgiven as you watch the Normandy shear closely over the dangers beyond the Omega-4 relay; the moment you realize that what is coming up - whether its death or victory, happiness or sadness - is all because of your actions and choices.
It's because of you.