I've already written my thoughts about the ending in the main thread, so I'm not going to post it again here. But I've heard people put this game's writing on the level of PST, and as an old school storyfag I have to disagree.
PST's writing is a level above Witcher's, not because of mechanical execution - I think Witcher's dialogue is easily on par - but in narrative construction:
Thematic consistency: PST had a focused story built around a powerful theme, and carried this theme all the way to the end. You begin as a man oblivious to, but haunted by, his past. Through the course of the narrative, you find out what this past is, and gather a group of allies who are themselves haunted by their pasts - pasts that, it turns out, are fundamentally intertwined with your own. As you near the end of the story, you are faced with the same choices and the same dilemmas that your past incarnations were faced with, and for a while it seem as though you can never escape it. Indeed, one of the most powerful moments in the game is when TTO tells you that all your companions are dead because they put their trust in you, thereby making you out to be another version of the Practical Incarnation. Yet, through belief - the force which has the power to unmake the planes themselves - you triumph against TTO, and your reward is simple self-realization in the form of your name, which you've gotten back now that you've come to terms with your past and are ready to take responsibility for your own actions.
By contrast, the Witcher games also start out with a fairly powerful theme: you can save people from monsters, but you can't save them from their own monstrosity. It builds upon this theme through showing you a land devastated by war, in which men are as responsible for the suffering as any monster: eg the Baron's brutal treatment of his wife, Radovid burning innocents at the stake, villagers sacrificing their own children for a decent harvest, Emhyr's cold and ruthless campaign, the Skellige clans' willingness to murder each other for the crown, etc. The problem, however, is that the game fails to take this theme to its logical conclusion. Instead, it falls back on a "save the world" story in which big bad Eredin is trying to destroy Ciri for inexplicable cause, and great old Avallach is trying to protect her so that she's able to save us all from the White Frost. In doing so, the game betrays its own themes and fails to keep its own ideas coherent. With pure & selfless characters on one side, and cliche evil villains on the other, the whole talk about humanity being its own monsters goes straight down the drain.
Character development: PST has a rich set of characters. All of them have flaws that tie into the story. Morte, for example, is a liar who lies to save his own skin, even when it's harmful to others, including the protagonist. Annah distrusts others and is difficult to get along with, but is stubbornly loyal to those who she does befriend, to a fault. Dak'kon, though wise on the surface, is tortured by the philosophical contradictions of his faith, and is enslaved by his personal honor. Fall-from-grace, in abandoning her nature as a succubus, has had to suppress her emotions, which becomes torturous as she comes to love TNO. The flaws of each of these characters are revealed to you through the course of the game, as your relationship with them deepen, and are not merely side quirks, but play into the bigger story of redemption and self-discovery that is at the heart of PST, as the player learns his own role in the suffering of others. PST also has several of the best villains that's ever graced RPGs: Ravel, Trias, the Practical and Paranoid Incarnations, The Transcendent One, and so on, whose motivations are both plausible and, in the case of Ravel, Trias, and the incarnations, tragic and sympathetic.
The Witcher also has a rich set of characters. All of them also have flaws. Triss is deeply in love with Geralt, but feels compelled to give him up because of her loyalty to her friends. Yennefer is torn between her desire to live a peaceful life and her political ambitions. Ciri is also torn - between her destiny at the side of her birth father, and her calling at the side of her adoptive father. Rouche is caught between patriotism and compromise. Djikstra on the other side of the same. The problem, however, is that besides Ciri, you barely ever explore these characters' inner turmoil. You never deal with Triss's unrequited love, except by choosing her, in which case the whole problem is forgotten. You never deal with Yennefer's conflict, except by agreeing to retire with her, in which case again, all's forgotten. All the while Zolten, Dandelion, etc. are just there for comic relief and the antagonists are among the worst, most cliche bads you ever face in a game. There is barely any character development, not even for Geralt, and certainly not for the villains.
Depth of ideas: don't get me wrong, medieval fantasy is fundamentally disadvantaged vs. the Torment universe in depth of ideas because the tropes in medieval fantasy are simply cliche when put besides the New Weird of Sigil. I don't think I need to even give examples in this case. That said, there are still themes in medieval fantasy worth exploring, and initially, as stated in the previous section, the Witcher explores them - what is the role of a witcher in a world in which men are as monstrous as monsters? How do you judge between two groups, both of which are just trying to survive? Do you have the right to take away people's comfort in their own superstitious beliefs? Does making moral decisions always equal sticking by your gut?
The problem, however, is that none of these ideas are ever taken to their logical conclusion. Whereas PST both asks "what can change the nature of a man?" and develops it into an answer, the Witcher never develops its themes much past the surface. Instead, we are given convenient black and white archetypes at the end of the game: Wild Hunt evil, Geralt & friends righteous; Radovid/Emhyr/Djikstra as ruler bad, Ciri as ruler awesome; Eredin power-hungry regicide, Avallach noble elven sage. Even what depth Sapowski put into these characters is lost as the game, in its final act, effectively divides its cast into moral opposites, while the whole "what is a monster, anyway" idea is forgotten altogether.
To end: PST is still the best written RPG ever. Even though in terms of dialogue construction, cinematic execution, voice acting, Witcher easily out performs it, when it comes to thematic consistency, character development, and depth of ideas, PST wins every time.
