Mass Effect 2 strives for noticeable improvements at the cost of some jarring omissions from its wonderful predecessor.
If you don't particularly care for RPGs in general you can more than likely disregard the majority of the complaints and criticisms Mass Effect 2 receives in regard to the things it changes from its predecessor. Mass Effect 2 in general features significantly improved action, faster combat, a better cover system, and more meaningful and interactive conversation options. It unfortunately sabotages many of the elements that made the Mass Effect series feel like more than just another "good third person action game".
So what exactly does Mass Effect 2 do wrong, if everything it has done so far seems to have been so right? The original Mass Effect feels much more like an RPG than the second, and in the process of "fixing" the issues many had with the original, they opted to simply remove them. Exploration, once touted as a significant element of Mass Effect's gameplay is all but nonexistent in this sequel. That's not to say that what is featured in Mass Effect 2 is of substandard quality, but it's a huge blow to the game's world.
It's difficult to describe, but being able to drive around random planets in the original Mass Effect's Mako vehicle was a remarkable experience. The fact that a majority of the planets were flat, or insanely mountainous, barren wastelands is quite frankly, irrelevant. The notion of exploring the unknown in the universe is irreplaceable, and it's sorely missed in Mass Effect 2. While the whole foundation of Mass Effect 2 is incredibly solid for the typical game, the removal of optional free exploration leaves an unfulfillable void that will sadden anyone who enjoyed the exploration aspect of the original.
One of the most disappointing changes in Mass Effect 2 is that it no longer has sidequests that feel meaningful. While Mass Effect 1's sidequests were not perfect, often featuring the same building structures copied and pasted over multiple worlds, the planets you got to roam were at always unique enough to be fun to explore en route to the sidequest destination. The most significant aspect of these sidequests however, is that they almost always featured some sort of unique moral choice or dilemma that had to be resolved at the end of the mission. These types of missions are completely absent in Mass Effect 2, and there absence is felt every time you do a sidequest and realize that the only objective is just clearing out a random, linear environment full of enemies with no dialogue and no choices. The lack of explorable planets hurt, but the lack of any depth and meaning to the sidequests hurts even more.
Loot has been completely discarded from Mass Effect 2 in the traditional sense, but you will still occasionally come across credits and new weapons from time to time. The lack of Mass Effect 1's cumbersome inventory isn't missed, but the inability to customize weapons and armour in the same capacity as the first game is somewhat upsetting. The majority of these things were re-allocated to different areas of the game, mainly as research. Rather than adding mods to your weapons and armour, upgrades are found and bought during missions which can then be upgraded in the Normandy's tech lab. Different ammunition types are now featured as actual powers tied to characters and different classes, so you're only able to access them if you or your allies have them as an ability. Thankfully you at least are no longer required to sift through meaningless hordes of generic armour and weapon replicas to find the ones with the best stats, when they all functioned largely the same anyway with minimal differences. On the surface it may seem like there's less weapons in Mass Effect 2, but in reality there's probably a lot more when you consider the fact that they all have unique properties and functions.
The classes have featured significant overhauls from the previous game, and your squad mates have suffered the most significant blow. Your allies have significantly less things to spend their attribute points on than they did before, and they receive much less points as well. The good news is that their powers have been streamlined somewhat so that the abilities that they do have are all generally useful. It's a bit disappointing that the available powers for your allies have decreased so significantly though. Shepard, the player character on the other hand still retains a handful of upgradeable powers and abilities, similar in scope to the first game but significantly streamlined as well. A unique new addition is that powers can be specialized once they attain max level. Usually this consists of choosing between more attack power or a greater area of effect. There's never really a correct answer for which upgrade is better; it depends entirely on whether you'd rather do insane damage to one enemy at a time or significant damage to all enemies within a small radius. Passive abilities usually have you choosing between increased survivability versus increased weapon damage output.
One of the most enjoyable and universally-loved changes to Mass Effect 2 is the addition of the conversation "interrupt" system. Assuming you've acquired the necessary paragon or renegade points, in certain situations you can now "interrupt" a character during a conversation to perform an unknown action. Paragon interrupts are generally helpful, or save someone from danger, while Renegade interrupts consist of punching people in the face, shooting bad guys while they're delivering an overzealous speech, and all sorts of awesome badassery. Perhaps most interesting of all, is that performing interrupts is never required and not always necessarily the best course of action. Sometimes disregarding them can reveal additional, perhaps even preferable dialogue options that would have otherwise been unavailable.
The game's engine has received a noticeable upgrade. The graphics are sharper, cleaner, and generally just a lot better looking. The character models have all been noticeably upgraded as well. The issue of hi-res textures sometimes taking their sweet time to load properly has thankfully been completely annihilated, so you will no longer witness odd sequences of extremely low-res textures for a few seconds while the game loads them. Voice acting is still phenomenal, and the sound effects are great. The game's music is good, but seems to go for a different sound than its predecessor. It's much less evocative of classic sci-fi than the first game but hints and motifs of Mass Effect 1's soundtrack can still be heard from time to time.
At the end of the day, Mass Effect 2 is probably a "better" game than its forebear. From a technical standpoint at least, and for the "action gamer" they'll most likely prefer Mass Effect 2's streamlined features and focus on combat. It's unfortunate then, for the Mass Effect 1 enthusiasts, that you can't escape the feeling of what "could" have been had so many elements of the original game not been lost in transition to the sequel. Improved vehicle exploration, weapon and equipment customization, larger skill trees, huge decisions that were supposed to greatly impact the next game are all but a distant memory of "what could have been?" It's disappointing that so many of the things that were promised in Mass Effect 1 were not able to be fully realized and further developed in what sometimes feels like a side story sequel. They simply vanished. That's disappointing, and being consistently funneled down linear environments as opposed to exploring a vast unknown galaxy is sometimes a hard pill to swallow.
Despite these disappointments, not all is lost for Mass Effect 2. It serves a useful and important function: it fleshes out the world you're living in. The Mass Effect universe is huge, and perhaps this side story sequel was a necessary stopgap to show the player what is truly at stake when it eventually comes time to save the universe yet again in Mass Effect 3. Hopefully it can deliver on the promises and ideas set up in the original game. It's disappointing that Mass Effect 2 can't quite seem shake that feeling of being "just another good third person action game", but it would be doing the game a disservice to not properly recognize the things that it does do so well. Despite Mass Effect 2's disappointments that are only really relevant if you played the first game, it's still a great game, and it's one that is definitely worth playing. Don't miss it.