(Thorough, well-written, spoiler-free review here.)
Following the opening action sequence, you are put through the usual paces of creating your character. This means choosing your combat class, among other things, and the classes from which you can choose will be of no surprise to any veteran of the prequel. You've got the Soldier class that can use just about any kind of firearm; you've got the Adept class that specializes in biotics (or "the force," if you will) and has virtually no skill with firearms; you've got the Engineer class that specializes in screwing with enemies' hardware and is similarly inept with firearms; and then there are three other classes that mix-n'-match skills from the three base classes.
What may be surprising to players who have mastered Mass Effect 1 is that the undisputed bad-ass class, the Vanguard (biotics and shotgun specialist), no longer reigns supreme. Weapons have been rebalanced, resulting in shotguns that are exclusively for use at short range (no more sniping with your Katana VII). The Vanguard remains a powerhouse in close-quarters, but the absence of any inherent defensive abilities coupled with the pitiful amount of shotgun ammo one can carry mean that if you put your Vanguard too near the action, you'll be dead rather quickly. Instead, you'll probably be using the new submachine gun at mid-range, most of the time.
Rather than coming across hundreds – or even thousands – of weapons of every description in your journey, you will come across two or three of each type. Any significant improvements to your arsenal will come in the form of upgrades which can either be bought at stores for exhorbitant prices, or researched in the lab aboard your ship, using research data found on missions and minerals acquired from laboriously scouring the surface of every uninhabited planet that you come across. These upgrades range from the standard stopping-power increase, to augmented reserve ammo capacity, to improved shield penetration, and are automatically applied to every weapon of the same type, whether used by yourself or by your team.
Most characters and classes have the ability to use a special type of ammunition with their equipped weapon. Your special ammunition never wears off, never runs out, and always kicks substantially more ass than your default ammunition. Consequently, you will put incendiary ammo on, and never take it off. I had cryo ammo as well, but I never encountered a single situation in which freezing the enemy was preferable to burning them to a crisp. The fact that there are no detrimental effects from using it, and that one ammo type is viable in all strategic circumstances, makes me think that the special ammo would have worked better as an upgrade than as a power.
Upgrades to your armour can be accrued in the same way as weapon upgrades. The option also exists to change out different pieces of it for minor enhancements to different stats (health, shields, melee strength, etc). There is no way to review your stat changes, however, so making the most of your customized armour set is difficult. You can also change the appearance of your armour, using a couple of preset patterns and a wide pallet of colours. It's a nice touch, but it feels like it might have been an afterthought; I wanted to colour my armour black with orange detail, but found that although there are three distinct variations on yellow, there is not a single detail colour that could be categorized as orange.
Hacking has been revitalized. No more will you have to play that stupid button-pushing minigame. It has been broken down into 'hacking' and 'bypassing'. For hacking, you are shown a box containing lines of differently aligned and coloured code. You have to pick that box of code out of a scrolling list, while avoiding the 'firewalls' that pop up. You have to do this three times for each hack. Basically, it's a pattern recognition game. Bypassing is much simpler, requiring you to connect nodes on a circuit board according to the symbol on each of them, and it plays like a simplified version of 'Memory'.
Your motivation in Mass Effect 2 is to prepare for a "suicide mission" (this is not a spoiler; "suicide mission" is written right on the box), your goal being to succeed at said mission, survive it, and to keep as many members of your team alive as possible in the process. Preparations involve acquiring as many of the upgrades that you deem appropriate as possible, recruiting every individual on whom you are given a dossier, and then earning the loyalty of those individuals by helping them resolve the one, convenient personal crisis that they are facing. The loyalty system seems rather contrived, but it functions pretty well as a means of forcing the player to bond with the characters.
And bond you shall. If you are not sufficiently prepared when you undertake your mission (or even if you take too long to prepare yourself), you will lose teammates, and you will feel personally responsible for every loss. On my first playthrough, I was unprepared and I was late, resulting in an ending that left me thoroughly depressed. (Note: It is possible to strike a balance between alacrity and haste that results in an entirely positive ending, but you will not achieve it on your first attempt.)
You will recruit a much larger team than you had in Mass Effect 1, with personalities covering a broad moral spectrum and unique skills and behaviours that make each of them invaluable in complementing certain playing styles. All of your surviving compatriots from your war against Saren will make an appearance, and a few of them will even take up arms alongside you once again.
Possibilities for pursuing romance with your teammates have also been expanded. Bioware have caved in and given the players what they want (even if certain players won't admit it): The new rule is, if it is organic, at least remotely female, and on your ship, you can have sex with it. (People playing with a female Commander Shepard may have a different experience.) However, sex-scenes, for all intents and purposes, have been removed. Rather than having a few seconds of bare-assed bodies on your screen, you are restricted to some passionate kissing and caressing before being shut out by a blacked out screen.
If you are, like myself, a massive nerd, you will be delighted to have an opportunity to experience firsthand the cultures of various species in the world of Mass Effect. I don't want to give anything away, but those few missions that totally immerse you in the traditions, history and politics of other species create some of the most memorable moments in the game.
The combat system is much improved over that of Mass Effect 1. Weapons are much more specialized and characters have much smaller skill trees, severely limiting the number of tactical options at your disposal, and forcing you to make the most of what you have. Using one power puts all of your powers into cooldown, meaning that you cannot use one super-powerful ability, and then use lesser abilities while you wait for your next chance to use it. Rapid-fire weapons have bonuses against shields, while single-shot weapons are most effective against armour. You are now required to eject heat sinks from your firearm and insert fresh ones (ie reload). Using cover is much more fluid; firing weapons is much more tactile. Squad commands are now essential for gaining a tactical advantage. Every shot, power, and order has to be carefully evaluated for maximum effect against often overwhelming enemies.
But it's not all good news. The cover system is buggy and inconsistent: If you try to vault a piece of low cover that, for no clear reason, is unvaultable, you will instead pop out of cover, leaving you unnecessarily exposed. Conversely, as 'take cover' and 'vault cover' are mapped to the same button, when you think you are getting into cover, you will sometimes instead vault right over it and into enemy fire. Finally, even the lowest difficulty can be unreasonably challenging. I had no problem with it, but developers should keep in mind that "every game is somebody's first game". Newbs are clearly not welcome in Mass Effect 2, and that is unfortunate.
In order to research weapon or armour upgrades, you need to have minerals, and the only way to acquire enough minerals to complete a substantial research project is to scan planets. The process of scanning planets involves holding down the left trigger, moving a tiny, agonizingly slow reticule over the surface of a planet, watching the accompanying graph for any indication of minerals, and then launching a probe when you find something. And yes, it is as boring as it sounds. If you hope to make any headway in terms of upgrades, you will have to spend hours doing this. And I assure you, your left trigger finger will give out long before you are done.
An annoying addition to the game is fuel. Using Mass Relays and cruising around a solar system don't require fuel, but traveling from one system to another within the same cluster will cost you. If you run out of fuel, you will lose your hard-earned minerals, and that is simply unacceptable to anyone who has spent hours hoarding them. Fuel is relatively inexpensive, but the cost adds up very quickly if you want to do a lot of exploring. Towards the end, as you start to run out of optional missions, you will be trying to weigh the potential rewards of continued exploration against the credits you would otherwise be able to spend on one last upgrade to take into the suicide mission. Far too often you will burn up valuable resources getting to a system, only to find that there is nothing there but more planets to scan and probe for more god damn minerals.
Dialogue now features 'interrupts'. At certain points in conversation, you will be prompted to pull a trigger. The left trigger corresponds to the Paragon alignment; the right trigger to Renegade. Should you choose to follow the prompt, Shepard interrupts the dialogue by performing some spectacular action that falls into the prescribed moral alignment. There are a couple of problems with this, though. First, you have no way of knowing what Shepard's action will be, only that it will vaguely follow the chosen moral path. Second, you never have to choose between the two alignments. You will only ever be prompted for one of the triggers at a time. Third, I don't want to interrupt the dialogue! I want to listen to every word, and so will you. Effectively, all the interrupt system does is add quick-time events to conversation, which is not a huge step forward.
The story continues on from Mass Effect 1, and its biggest accomplishment is filling in one of the massive plot holes of the previous game. I can only assume that Mass Effect 3 will fill in the rest. I won't say any more.
There are a few spelling problems ("adpted" instead of "adapted"), the dialogue system gets sloppy (giving you the option, "ask about..." rather than presenting a shorter version of the question), and your Journal will sometimes have a little hiccough (telling you that you took the Renegade option on a mission when you actually took the Paragon option), but those things are par for the course in a game of this scope. This game is so big, it comes on two discs – one disc for the opening and ending, the other for everything in between, meaning you only ever have to change discs twice in one playthrough.
But this brings me to my main complaint: IT'S TOO FREAKING LONG! One playthrough, completing every optional mission I could find, took me 65 hours and 49 minutes. This is a game that is meant to be played repeatedly – try a different class, try a different gender, try a different attitude. The player is meant to try so many different things, but when each try takes 65 hours, there is no way that anyone is going to find the time. Bioware even acknowledge the fact that most people don't have the time to play their game repeatedly in the way the set up the Achievements: Achievements for using specific powers a set number of times can be unlocked for ordering your squadmates to use those powers. "We know you aren't going to play it again just to be a different class," they say, "so here, have the Achievements for taking different teammates on missions." Make your game shorter! The more times I can play it, the more enjoyment I will get out of it in the long run.
Is Mass Effect 2 fun? Yes...most of the time. Does Mass Effect 2 get tedious? Definitely. This is a game that doesn't so much reward patience as it does loyalty. Those who go into this game knowing that the bad parts are worth it for the good ones will be greatly rewarded. Those who do not immediately fall in love with the meticulously crafted world and brilliantly executed narrative probably never will. One way or another, this is not a game out of which you will be getting a lot of play. Enjoy it once, and then start waiting for the epic conclusion in Mass Effect 3.