The second attempt in this impending trilogy has embraced many changes and it works really well. A long review.
When you begin, you have the option of starting with a brand new character or importing a character from Mass Effect 1. The latter is a much better and satisfying experience since the impact of the decisions you made in the first game can range from subtle to surprising. As you progress through the 2nd game, you'll realize just how different the game plays out as you somehow come across old characters. In my own particular experience, my love interest from the first game turned sour in the 2nd game and the defiant Krogan, Urdnot Wrex, who was formerly my crew member is now a leader of a Krogan Clan that vouches for me in front of other Krogan. This was the same Korgan I almost decided to put a bullet to his head since he almost compromised the main mission from the 1st game. Keeping him alive sounds like a win-win situation but despite his good intentions and his loyalty to Shepard, his agenda is large in scale and opening the possibility that something major can go wrong.
In Mass Effect 2, Bioware has done an excellent job at providing a deeper and emotional connection to your characters - some of which will get a chance to rejoin your new crew. While some decisions are more novel, the game wants you to feel that your decisions are doing something to the universe. To bring more attention to character and the narrative, it seems that Bioware has made many changes to the gameplay elements to emphasize this new direction.
THE NEW LEVELING SYSTEM
Compared to Mass Effect 1's level system, RPG-like leveling has been simplified to a few handful of talents. These have their pros and cons. In Mass Effect 1, Shepard had as many as 8-10 talents to tinker with while maxing out at level 60. Further advancing talent was a simple point per talent advancement system. Character talent progression was fast paced and many RPG-heads (myself included) enjoyed the tweaking. However, the talent to Charm and Intimidate NPCs was based on the points you assign to your character, irregardless of how reckless or thoughtful you make of your decisions. In Mass Effect 2, your ability to charm or intimidate characters are based on charming or intimidating characters. Makes sense. This is more pronounced if you're more inclined to pull the trigger during the interruption scenes. On the flipside, the simplified leveling makes your character feel more generic but allows your character to be proficient in the use of all class weapons at the beginning of the game. This "lack of flexibility" seems to be by design since it indirectly encourages multiple playthroughs with other classes. While the meticulous character leveling from the first game will be missed, it becomes more apparent why these design decisions were made as you reach the final battles of the game.
NO MORE CUSTOMIZING. JUST UPGRADING.
Customizing weaponry, ammo or armor with add-ons has moved on. Weapons and armor are no longer class specific. Instead, upgrades to weapons and armor are handled through the leveling system and researching. Upgrading Biotics and Technology also work the same way. No longer will you have to sift through a locker or hack a weapons station. Instead, you will be combing terminals and spare parts that will be researched back in the Normandy. But since you're researching under Cerberus and not under the Alliance, researching costs money and raw materials such as Platinum, Iridium, etc. Having money in Mass Effect 2 seems to be plentiful but gathering the raw materials can be tedious via planet probing. Fortunately, finding raw materials isn't difficult since there are plenty of planets rich in minerals but you will definitely spend some considerable time mining them. By default, your crew will be carrying the latest and greatest stuff and customizing has been limited to selecting the right weapons.
TACTICS
Tactics has taken a front seat from the last game. You can now position each of your team members individually as well as assign hot buttons to their talents without freezing the action. This works great as long there isn't multiple targets that move too quickly. Along with the standard cover system prevalent in today's popular 3rd person shooters, climbing over cover has become the norm.
Despite these minor updates, the biggest change is the classes themselves. The class you choose radically changes the way you approach enemies and missions. While Mass Effect 1's classes are merely statistics leaning towards a particular set of talents, Mass Effect 2's classes are more specialized and the gameplay can differ greatly by the type of class you choose. For instance, the Vanguard class relies heavily quickly closing in on an enemy while the Infiltrator uses stealth and invisibility to avoid direct conflict. The classes also have very specific talents. The Soldier Class can use the Adrenaline Rush talent to make the battlefield appear in slow-mo for effective shootouts. Each class handles their business differently on the battlefield and this a welcome change.
CHARACTERS AND NARRATIVE
Cookie cutter side-missions with the slow moving Mako are gone and replaced with "missions" from crew members asking favors from Shepard to handle some personal business that may or may not help with the main storyline. How these side missions pan out will play heavily into the dynamics of your crew as you try to earn the loyalty of each of your recruits. The side missions are also much more exciting and varied. One side mission will lead you to a courtroom and you representing one of your crew as an attorney. Another has you involved in a mission of vengeance and the only thing stopping one of your own guys from sniping his target for whatever reason is you. In the end, the level of compassion you have for your crew will ultimately play a major part on the suicide mission. Loyalty may even get your crew member to do something unexpected or something completely hilarious - just ask Mordin.
Mass Effect 2 likes to play with the characters they create and it works well. Their behaviors are justified and not just added for shock value.
Characters seem more brooding while others are annoying and downright nasty in their approach; but they can fight! It's interesting since the game's eclectic cast is somewhat a direct extension of Shepard's likeness based on the choices you make. It ultimately makes you care about them no matter how annoying they may be.
Storywise, the choices play an even larger part as the game nears the end. Choosing sets of teams to tackle the larger scale battles is not an easy decision. Imagine the complexity of your decisions if weapons management and talent system stayed similar to Mass Effect 1. The pacing would be horrible. Keeping the crew alive will consist of choosing the right teammate for the right job. In these situation where time is of the essence, the only thing you should be worrying about is who is going with you.
OVERALL
Aside from its minor negatives of planet mining and generic hacking screens, Mass Effect 2 is more easily identifiable and is a completionist's dream. If you're the type of person who prefers the grind and micromanagement of their characters, you may miss the old Mass Effect. However, if you lean more towards action with some witty dialog and immense replay value, then this game is for you. The bold changes that were made from the first game are a welcome addition that not only make the game accessible to more types of players but to provide an experience in a very intriguing universe we know as Mass Effect.