So how bad does it suck? Sum up in a few short sentences pls.
1 - Setting is barely developed beyond than "Numenera anything goes.", it is weird for weird sake instead of building a rationale behind that weirdness. You won't find the Sensate HQ or Dustmen Mortuary type of location here. You feel alienated of the whole thing.
2 - Everything is painstakingly explained to you on lore dumps more than one time and as there isn't a proper rationale behind it all like the Dustmen faith, Sigil as the city of the doors, alligment determining the plane you belong to, the Lady of Pain and all of that, you don't have enough of a context to make sense of all that deep lore that kinda become irrelevant and make talking with the NPCs a pain.
3 - Your companions lack alot on the interactive department, a Dakkon kinda of moment where you go on a deep conversation about his faith and his understanding and doubts about it grounded on Planescape lore won't happen ever. Like Fall of Grace moment where she kisses you and you die because she is still a succubus even if a cast one or deciding what to do with Morte and the Pillar of skulls. How about Vhailor and you convincing him he doesn't exist? There isn't enough context and interaction with the companions, they are one note characters that are barely connected with their enviroments, don't believe on anything, don't desire anything and are boring.
4 - Everything feels random, like a typical theme park setting where nothing connects with nothing.
5 - There is major sloppiness on the main plotline delivery, they just brush off on the necessity of giving you a plausible motivation, on the necessity of introducing you slowly to this setting and just bombard you with out of context exposition, creating a feeling of alienation with the whole thing making it impossible for you to immerse on the game. Things start getting worse as you proceed and Deus Ex Machina moments start being pulled out of ass without any beforehand explanation.
6 - The tides system was butchered and there is only a vestigial form of it on the game killing one of the few things that could give some structure of rules on this barely developed setting.
7 - There aren't strong characters anywhere on this game, there isn't a Ravel or a Trias moment on this game. You hear alot about the characters but on the actual interaction with you, they are as straight as you can get. Don't expect a Ravel kind of wits battle here. The characters you know very little about will just regurgitate alot of exposition on you, the game will assume you care then something will happen to push the plot forward. The End.
8 - There is very little creative reactivity that Torment was known for.
9 - There are very few actual quests on this game, with beginning, middle and end, most "quests" are actually minor meaningless encounters. Not that Planescape Torment didn't have small encounters but it had extensive story arcs too. You go doing minor tasks after minor tasks without anything relating with anything and feel disconnected of the whole thing.