Released with no fanfare in December 1999, Planescape: Torment was one of the best games of the year largely on the strength of its one-of-a-kind story about immortality and human nature. Even though it wasn't a commercial success, I believe it's one of the most influential role-playing games of the last decade and ideas it pioneered have since found their way into much better-known games (mostly from BioWare), such as Star Wars: Knights of the Old Republic and Dragon Age: Origins. Apart from the overall quality, size, and depth of the game's ambitious story, on the surface Torment was basically just another Infinity Engine game, a "total conversion" of Baldur's Gate with a weird cast of characters and an awesome voice cast. But even though Torment relied on the conventional mechanics of the time, I believe it introduced a radical idea that's had a profound influence on story in Western RPGs: player dialogue with subtext.
In other words, Torment is the first RPG to introduce player intent into dialogue, which may be contradictory to the substance of the dialogue -- it's a game in which you can say one thing and mean another, and use this to deliberately lie at times, by means of the authored choices presented to you.
In a typical RPG, you might be asked by a character to retrieve an item, and tell that person "Yes, I'll do it" even if you as a player don't really know if you're going to do it or not -- probably you just want the quest logged in case you stumble upon it. You don't think about these types of interactions, and, as evidenced by the completely disposable text content for quests in games like World of Warcraft, they do little to build a meaningful connection between you, your character, or the gameworld. But in this same type of situation, Torment typically would give you at least two options: "Yes, I'll do it" (Truth) and "Yes, I'll do it" (Lie). And it would fully support these choices -- lying would affect your character's moral alignment, leading to other changes in gameplay. But even when it didn't really matter whether you told the truth or not, the game made you stop and think about what you were saying.
I remember encountering this type of choice in the game for the first time and feeling frozen in my seat -- never before had I considered my options so carefully around a straightforward dialogue choice. Some previous RPGs, such as Fallout, might have let me play as a smartass, who might make a sarcastic quip instead of answering a question directly. But no game prior to Torment made me feel like I could, if I so chose, look another person in the eye and lie to him with a straight face.
Plenty of RPGs before and after let you play as morally evil characters, letting you randomly kill people if you want, usually just after saying something terrible to them. But this type of psychopathic villainy, while potentially exciting for a little while, tends to feel hollow because your ability to empathize with your character rapidly breaks down as you start massacring entire towns and looting people's corpses for their boots and gold. It's out-of-character and not the "right" way to play, even if it doesn't outright break the game. On the other hand, in Torment you could do some really deceptive, scheming stuff simply through talking to people -- and it made you feel connected and close to that evil, because everyone knows what it feels like to lie, and in Torment you have reasons not to be up front with everyone you meet. As well, the option of lying made the default act of telling the truth feel more consequential. By letting you get inside your character's head as well as your own head, Torment created such a strong connection with players like me.
It took years for anyone to improve on Torment's basic idea of letting you determine your character's intent and not just the content of his speech, but the Mass Effect games finally took this a step further through their system of subtext-driven dialogue. In those games, it's genuinely exciting to see the protagonist act out your dialogue choices, because you don't know exactly what Shepard is going to say or do -- you tend to just pick the emotion or intent behind the outcome you want, and your reward is seeing Shepard act it out faithfully, in most cases. The basic consequence of this is that dialogue sequences tend to be worth watching and listening to, something that's rather important for a game that's bothering to have tons of story content.