One of the most common traps that science fiction can fall into is the tendency to treat aliens in an overly basic manner. It happens all too often; we are introduced to a species who are identified by, say, their warrior instinct or attuned sense of honor. This would be fine except for the fact that most science fiction writers tend to make this trait the defining characteristic of every member of that race. The leader of the tribe? Dedicated warrior. The old grandma who takes care of the kids? Waxes eloquent with war stories of her grandsons. Baby alien? Wants to grow up to be a noble hero, just like his dad, brothers, uncle, grandfathers, etc etc. The entire species becomes subservient to these general and stereotypical character attributes and, once you see one, you know immediately what to expect with dreary regularity.
Star Wars: An Exercise in Single Dimensions
The Star Wars series is a classic example of this. It maintains a sci-fi universe chock full of diverse alien species. In the more recent prequel movies, it gets to the point where any public gathering that you see contains a dozen new aliens that you've never seen before, products of George Lucas' artistic expression given an absurd amount of free reign. But, when you think about it, these aliens are a collage of mind-numbing simplicity.
Take Chewbacca and the Wookiees. He is steadfast, loyal, strong, and ferocious to those who are his enemies. While he is one of the only members of his species that we ever see, we find out from Han that his entire species is essentially full of Chewies. Their loyalty is cemented by the cultural concept of the Wookiee 'life-debt', and they are altogether identified by their hairiness and fortitude. There are no voiced or visible exceptions to the rule.
How about the Jawas or the Ewoks? Each are small species that are all subservient to their assigned traits with little to no diversity or uniqueness outside of them. The Jawas are cowardly merchant scavengers and that is all you ever see of them. The Ewoks are cutesy tenacious tribal teddy bears. Tusken raiders? Perpetually angry, nomadic... raiders. Remember those long necked svelte white aliens from the second prequel (Kaminoans, for the nerdy)? All of them were defined by a cool scientific attitude and apparently their entire race was dedicated to cloning people. The only possible exception I could think of was Jar Jar Binks being distinctive from the rest of his Gungan people. Unique in the worst way possible.
The point is that you never saw an Ewok who acted apart from the mold. You never saw a Jawa casually toting a laser rifle. You never saw a cowardly Wookiee. In Star Wars' defense, this simplification of entire species was probably necessary given the sheer amount of them. But it seems very limiting and not at all true to reality. After all, you can't label humans simply as aggressive imperialist impulsive warmongers. If you think that's correct, then you've never heard of Confucius, Marcus Aurelius, or Buddhist monks.
Battlestar Galactica: An Exercise in Bipolarity
The next step up is to classify your aliens in strict this-or-that terms. This is also flawed and unsophisticated, but it is a bit better. To do this is to comparably regard life as only having those who are Democrats and those who are Republicans. Sure, there's a significant population to both sides, but there are still people who exist outside of these ideologies. Another example is the knee-jerk reaction to look at everything in life as good or evil. Many things can be classified as such, but there's also a plethora of things that aren't quite so clear cut.
Battlestar Galactica gives us the Cylons who, while initially are just set on killing all the humans, gradually evolve into allowing the existence of a splinter faction: those who believe the humans can be spared and worked with. Some Cylons may look different, but all of them fit into these two categories and that becomes their most important characteristic: pro-human or anti-human. Any side quirks are subservient to this overall identifier. Battlestar Galactica makes an effort to give the Cylons some nuance. The Boomer models are known for being duplicitous. The Leoben models are known for being manipulative. But in the end, all that really matters is their alignment to the humans, making them two-dimensional at best.
Mass Effect: Aliens with Depth
Despite how easy it would be, Mass Effect avoids the sci-fi writers' trap in its totality. And you wouldn't quite think that going in. After all, the game's internal Codex (encyclopedia) identifies the separate species through some generalizations. The turians are opportunistic, imperialistic, and prone to find themselves in combat. The quarians go on pilgrimages and rites of passage, as well as generally being known for stealth. The asari are empathetic and known as diplomats and lovers.
The key difference here is that Mass Effect uses these stereotypes merely as guidelines. Much like regarding humans as impulsive, stereotypes have a grain of truth but aren't necessarily indicative of races or species as a whole. Mass Effect has countless examples of this. Sure, turians are known for being mean and aggressive, and yet you find one (Garrus) twisting those traits toward something you wouldn't expect, serving as a Robin Hood to a destitute population. Similarly, the crimelord Aria T'Loak (an asari) is a subversion of what you'd expect from the stereotypes; she's the ultimate socialite... of illegal enterprises.
I could go on, but the point is that Mass Effect depicts aliens in a way that uses established stereotypes as a foundation of understanding, a mere stepping stone to further depth instead of the end point. The aliens aren't all the same traits like in Star Wars, nor do they only tack to a bipolar definition like the Cylons of Battlestar Galactica. Instead, like us, they are capable of operating completely independent from what you might expect, while simultaneously allowing you to recognize what makes them a member of their unique species/race.
I still am not very far into Mass Effect 2, but I did want to point out this particular facet of science fiction storytelling that the game has excelled at so far. It gives me great hope in the continuing quality of this stellar story.