BioWare has taken a great game and improved on it with excellent results.
Mass Effect 2 takes place two years after the events of the first game. Commander Shepard is still out to stop the Reapers. I'm not going to go in-depth on the story, but know that to get the full experience of out Mass Effect 2, you need to play the first game. The Mass Effect universe is large and has great depth, and being thrust into it in the middle of the trilogy is not the way to go about it. Also, choices made in the first game carry over to this one. If you saved a certain character or let a certain character die in Mass Effect, those consequences carry over to Mass Effect 2. This system of carrying over information works great and really adds fluidity to the series' overall narrative.
Mass Effect 2 feels more like a third-person shooter than the first title, but it still definitely has its RPG roots at heart. Characters and squads are customizable and experience is earned at the completion of missions which lead to skill points being earned and these points can be assigned to skills to strengthen them. It's still very much an RPG, but almost all of the missions are shooting-oriented. In the first game, there would be entire side missions that involved simply talking to people or mediating an argument. These types of missions are far less common, which is unfortunate, but thankfully the combat works exceptionally well so everything is still a blast to play.
Mass Effect 2 also puts a lot of emphasis on your relationships with your squadmates. In the first game, for example, I never believed any of the romance options for a second. Here, however, i find everything to be believable. All the characters are fuller and the end result is that none of the relationships are unquestionable. Each squadmate recruited also has an obtainable mission where you could earn their loyalty, adding further depth to these characters. I felt as if I truly knew my squad after playing the game, which is an incredibly impressive feat for BioWare to have pulled off.
With presentation, Mass Effect 2 is near-perfect, only brought down by some bugs. The character designs are all excellent and the tech is gorgeous, but in some cut scenes, objects could go through other objects as if there was nothing there. For example, Shepard's arm might pop out from cover or he might drink something while wearing a helmet. Also, I noticed a few times where the cut scenes would freeze for a few seconds and then continue. There is also a very bad issue with small text; this game is really unplayable on an SDTV due to the text size. Other than these issues, which some might consider to be nitpicks, Mass Effect 2 is a gorgeous game.
In an audio sense, Mass Effect 2 is actually perfect. The music is simply excellent, and every single voice actor brings their A-game, no matter how small a role they play. The game is largely voice acted and the fact that the whole cast does so well is truly a miracle. The sound mixing issues from the first game are also gone. In the first title, music would play louder than dialogue on occasion, for example. In the sequel, BioWare makes sure you can hear every gloriously read line of dialogue.
The galaxy in Mass Effect 2 is also larger than in the first game. There are a few new species and more cities to explore. Fans of the original Citadel will be disappointed by the smaller version of the city that can be visited here, but it's still nice, even if it is smaller. Also, this is just a nitpick that bugs me also, there are no females of alien species. The Krogan, Turian, Drell, Batarian, Vorcha, Hanar, Volus, Elcor and Salarian species have virtually zero female representation in this game, which I find a little bit odd.
Like the Citadel, the way you play missions on uncharted worlds has changed. Now, you need to scan a planet to find mission instead of being told of an anomaly super-quickly. The scanning for missions is not bad, and it cuts out the horrible Mako sequences from the first game. however, scanning is truly dreadful when mining for elements to purchase upgrades for weapons, the ship, or biotic abilities. Each planet takes about five mintues for a thorough scan. It's truly excruciating.
Fortunately, these missions from scanning are infinitely better than those from the first game. In Mass Effect 1, all the missions felt the same and looked the same. Here, they are varied more and all feel different. They are better in virtually every single aspect. There are less of them, but that's completely excusable due to the loyalty mentions that were mentioned before. Each squadmate has an optional mission that can be completed to earn their loyalty, and each of these missions takes as long as a main story mission. If you choose to ignore these loyalty quests and the side quests, Mass Effect 2 could still take you well over 20 hours, and this could easily be doubled by doing everything the game has to offer.
If there was a word to describe Mass Effect 2, it would be "epic." The story, take my word on it, is fantastic. The way the characters play off one another is simply amazing. There are also plenty of twists and turns in the game, plus some really touching moments (especially in the loyalty missions) that elevate Mass Effect 2 from being simply a game to being an experience.