The finest accomplishment of the role-playing game, whether played with a controller or a keyboard (or even pen and paper), is its ability to pull the player into the game world. Playing an RPG is like taking a vacation to a foreign land, rich with its own culture, people and history. While gameplay is a significant part of any game in general, RPGs can make up for weak gameplay by having a powerful story, believable characters and a world you want to spend time in. My most recent RPG outings are Star Wars: The Old Republic, The Elder Scrolls V: Skyrim and Mass Effect 3. I am currently playing through Kingdoms of Amalur: Reckoning.
SW:TOR is hard to discuss here because an MMO is, by its nature, a different beast than its single-player brethren. Though I no longer play SW:TOR, it made a lasting impression on me. I played World of Warcraft for three and a half years (I retired my paladin almost as soon as I hit level cap in Cataclysm, real life distractions and all that nonsense). The particular element that SW:TOR exceeded in compared to WoW, is the immersion. SW:TOR's individual class story made you feel like you were changing the world around you. Seeing the effect caused by your actions is one of the most gratifying experiences in RPGs. WoW always seemed to lack that visual confirmation that your actions as a character in the game world meant anything.
An example of this in a single-player RPG is Kingdoms of Amalur. One quest has the player reuniting a war-widow and her missing-in-action husband.. Upon convincing the husband to return to his wife, he exclaims something along the lines of "I'll go to her right away!" and then proceeds to sit at the bar and nurse his mug of ale, as he was doing before learning his wife was home waiting for him. Moments like this make the player's actions feel insignificant.
In addition to the lack of cause and effect, dialogue can be one of the worst ways to break immersion. RPGs without branching dialogue systems are not confronted with the same set of problems others are. For instance, Skyrim and Kingdoms of Amalur: Reckoning are both solid games. However, they both have one distinct flaw that hinders immersion in either one.
Both games feature a branching dialogue system. The non-player character talks and the player is presented with a list of possible responses, then the NPC gives a scripted answer to your chosen line of conversation. While the branching dialogue in itself is an immersive factor, it lacks depth when only one of the parties is actually speaking. The silence as the player reads through the possible responses pulls you from the game in an abrupt way, never letting you fully settle into the game world.
This is where the Mass Effect games get it right. The branching dialogue system in Mass Effect works the same as the ones in Skyrim and Amalur. There's just one difference. The player character, Commander Shepard, is fully voiced. Hearing your side of the dialogue keeps you in the game and makes both your in-game persona and the NPC more authentic. Not only reading, but hearing the emotion in your character's voice helps you feel what they feel and, in the end, provide a better role-playing experience. When playing through Mass Effect 2 and 3, I grew emotionally attached to not only Shepard's companions, but to Shepard himself. The use of voice has immeasurable value in making the game world believable and fun.
The tiny details that go into immersion in a game, from graphics and sounds, to a clutter-free interface, all play a role in allowing the player to appreciate the world of the game. With the current generation of consoles as technologically powerful as they are, it feels awkward to have to read half the dialogue in a modern RPG. While the moments may be rare, RPG's are at their best when you cross the line between player and player character.