The "choices, challenges, and consequences that players face while navigating a virtual environment" mentioned in the text book are apt descriptors of
Mass Effect's gameplay. It can be broken into two basic categories: free roaming and cinematic. The free roaming gameplay is where the navigation and combat of the game take place. It is consistently played from the third person perspective in which the camera can be rotated around Commander Shepard in three dimensions but is usually at a fixed distance. The player runs about various environments, bound by the game's rules, and either shoots enemies or interacts with objects or NPCs with the A button, which frequently leads to the cinematic gameplay of
Mass Effect. This second plane of gameplay involves the "choices" while the combat embodies the "challenges." The cinematic gameplay mainly consists of interactive conversations that Shepard has with NPC characters. The player chooses what to say using the previously discussed interface. While surviving the free roaming gameplay can be difficult at times (especially on "Insane" difficulty), it is quite easy to get what you want through conversation, perhaps too easy. The game tries to balance the free choice experienced in the cinematic gameplay with the challenge of physical conflict. These battles primarily test the player's Spatial Awareness and Reaction Time. Defeating your immediate enemies and surviving the firefights triggered by the primary quest is an explicit challenge while implicit challenges can be set by the player for him/herself. For example, perhaps I will set the goal to be mean to the NPCs in all of my conversations. This is difficult for me to do (and possibly affects my real world behavior) but as it is not a direct objective set by the game, it is implicit.
Victory Conditions can be based on individual encounters but ultimately, winning in
Mass Effect is playing through the entire story. Loss Conditions are simple; every time you die, you lose, though you can keep playing by reloading a game. On a smaller scale the conditions may change. For example, in the altercation with Wrex on Virmire, the player might consider it a loss if they are unable to reason with the krogan verbally and he must be killed. On the other hand, they may feel it is a victory to be able convince him to remain on Shepard's side. These feelings the player has could be seen as "implicit" victory or loss, while completing the game or dying in the attempt are "explicit" victory or loss. Sometimes, a Trade-Off must be made. For example, you might implicitly lose a little to implicitly gain a lot. Letting a few hostages die on the hijacked asteroid might have you feeling bad about yourself, but at least the evil terrorist leader isn't free to run around the galaxy killing hundreds more. Much of what counts as positive or negative feedback depends on what the player wants out of
Mass Effect.
Intrinsic Knowledge, or knowledge given to the player by the game, is very important to
Mass Effect because it allows him/her to understand the plot and figure out what he/she has to do. Also, as is true with most games, the Time of
Mass Effect is "limited" to the player's location. The sun never sets and the weather never changes. This is because the scale of the levels (or planets/space stations) is not great enough that the player will spend days there. By contrast, in open-world games like
Grand Theft Auto 4, the player spends all of their time in one city and to make it feel authentic, it must go through the expected daily transformations. Extrinsic knowledge the player might want to bring with him/her are basic assumptions about the world of sci-fi. For example, if one makes contact with fast-moving, glowing beams of energy, one will die. It is also helpful to understand the rules of the universe such as physics. This will help you pilot the Mako.
Speaking of the Mako, there are also minigames and vehicle sections in
Mass Effect but the minigames do not advance the story and driving the Mako is similar to running around on foot but with more firepower and a greater chance of falling into a pit of lava or off a cliff.
The Interactivity Mode of
Mass Effect is Player-to-Game. There is no multiplayer and the player is confined to the environment of the game. While it does not involve other human players, the NPC antagonist and his followers have an opposing goal to the protagonist. Therefore, within the story, the game could be considered "Zero-sum." It is quite impossible for these two opposing forces to work together. If anything, the dynamic of
Mass Effect is Restorative because the static balance is upset in the beginning and Shepard is working to make things "orderly" once more.
The NPCs of Mass Effect, meaning the characters controlled by the game's AI, all fit into the "Fantasy" character type. Since I've already covered the personalities and story elements of the main co-protagonists and antagonists, I'll look at the characters of various species that populate the galaxy and do not have scripted reactions to your actions. Instead, they need to adapt to everything that happens immediately. In combat, the co-protagonists become this kind of NPC and assist you to the best of their ability. Often, this assistance includes running into your line of fire like secret service agents for your enemies or standing in the open and absorbing plasma when they should be taking cover. There is a squad command system which is pretty easy to use but it would be nice if you didn't have to worry about your allies so much. However, if you allow them to freely use their powers, carefully build up their skills, and properly manage their gear, they can really be quite helpful. It's always cool to be a millimeter from death only to be saved by a biotic throw from one of your teammates that sends your assailant tumbling away. 
Another type of NPCs are the friendly or neutral characters that just stand around as window dressing or heralds for sidequests. Depending on what planet you're on, about 15%-50% of these characters will be eligible for conversation and a handful of them will have quests, such as: "smuggle this merchandise for me" or "assassinate my competitors." These kinds of duties can be gained from the undercover criminal types and will lead Shepard down the path of a renegade. Other characters, like Admiral Hackett (only heard, never seen, and voiced by Lance Henriksen) will give you more noble goals, like: "take down the extremists while making sure not to hit the civilians." These talking NPCs have their dialogue scripted to respond to Shepard's queries the same way every time. They will not vary depending on the circumstances. The window dressing NPCs do not change either, eternally waving their arms like they're conducting an invisible orchestra and nodding their heads in silent conversation with their fellow NPCs. These quiet, nameless beings make the environments seem a little less dead but it's quite rare to find any that move and they provide no cacophony of noise to supplement their presence. There also seems to be only one gender of every species other than human. For the mono-gendered asari, this makes sense but why are there no turian chicks? 
Your enemies are all NPCs as well. They engage you more intelligently than your allies engage them but often aren't too sharp either. Fighting them would be more fun if they were a bit more varied. Basically, all you do is shoot and shoot until they fall down dead. There is no option to merely disable enemies and no more strategy involved in killing them. I would especially like to see some creativity implemented in the boss fights. All you do is hold down the trigger as Saren floats around firing missiles. For the most part, you fight geth, a robotic enemy. They make neat sounds when communicating to each other during combat and sometimes will make decisions that surprise you (I wish that all the enemies would be damaged by your gunfire, though; blood may fly out of them but there is no wound to indicate where it came from. I hate it when games do this, even though most of them do). Overall, the enemy AI is perfectly acceptable. I don't play the game for the combat. 
Concept art for the lead villain can be seen below. Notice the spines on the side of his head and robotic arm that make him clearly distinguishable from other members of his species. Other strategies Mass Effect uses to make NPCs stand out are unique colors, voices, and animations. 
One HUGE benefit that Mass Effect has is that all of the NPCs look great. It is a breath of fresh air after the sloppy and hideous characters from Elder Scrolls IV: Oblivion. I'm not suggesting that every character needs to be attractive (half the aliens would have to be cut from the game), but they need to look "good," which is a vague way of saying they should fit their roles. If a character is to be respected (like the comical elves of Oblivion were) they shouldn't look like clowns. Furthermore, they can't be stiff, stoic, and expressionless. Mass Effect's NPCs emote and gesture realistically, even if it is sometimes repetitive. 
