Western RPG games have to have goblins. It's a well known fact that there have to be kobolds, goblins, orcs (or Orks), elves and so on. If they aren't there, we just get a bit confused and run away. Planescape Torment doesn't have these things, yet it's one of the best RPGs you'll ever play, because it challenges our preconceptions and delivers the finest, most original world seen in the genre. One that's never been bettered and, in all likelihood, never will be.
Playing it was like reading a book and this isn't an exaggeration. The dialogue was so extensive it spanned over 5,000 pages, more than even famous tomes like In Search of Lost Time by Marcel Proust (which comes in at around 4,300 in the Modern Library translated version). It's a whopper and each character has so, so much to say. It's something that can put people with any sort of patience problems off playing.
What it does is help create a world rich and vivid, much more so than your average RPG goblin-infested land. It's full of weird and wonderful characters, intriguing concepts about the nature of one's actions having consequences beyond current existence, and is set in a world that is constantly throwing up surprises and unique set pieces.
It also perhaps embodies a time when games weren't just there to make money. The reason I say it may well never be bettered is that it's not profitable to allow a development team to really go for it, to experiment and to create a game that draws praise from literary, non-gaming sources. There are few publishing companies that would be willing to spend the money on voice talent for 5,000 pages worth of dialogue. A game based on a niche subject wouldn't make the money back, sadly, because people are too wary of new ideas, preferring to stick with their CODs and Halos.
It's a huge shame, because the world of Sigil is so exciting and interesting, it really deserves to be explored some more. A world where sorcerers are punished for their misdemeanours by being perpetually set ablaze, a world where the brickwork itself has a life of its own. One where your main companion is a floating skull that likes to crack jokes and where the usual trick of having an amnesiac main character is developed beyond what any game has done before.
The Nameless One is worth discussing in further detail, actually. He's the guy you play, waking up on a slab in a vast mortuary. Because the universe of Planescape is so singular, everything you do will be a new discovery, which is part of what makes the game so interesting to play. Every major character has an extensive back story, often if not always linked to the Nameless One in some fashion.
Your quest is to discover just you who are, why you're so covered in scars and tattoos and just why you ended up waking up on the mortuary slab. As you progress through the game, you'll meet up with and take on board into your party such characters as Morte (the floating, talking skull), Vhailor, an animated suit of armour with a penchant for dealing out divine justice, and Fall-From-Grace, a succubus who one can find in an 'intellectual brothel'.
These are just a few of the characters and demonstrate just how varied and intriguing the ideas in the game are. As mentioned, it's a world the likes of which you'll never have seen before and while it still uses traditional classes like rogues, fighters and spell casters, the places you conduct your actions in are so unique, it's difficult to resist droning on and on about them. I already feel like I've perhaps put a few too many snippets of information in this article as it is.
If you've played Baldur's Gate or games of that ilk, you'll know how Planescape: Torment operates. It's a traditional isometric-y RPG point-and-click with a unique storyline. It's also very hard, especially if you don't set your character up as a specialist. If you attempt to be a jack of all trades, you'll die, a lot. In fact, you'll be unlikely to be able to complete the game, as one section requires a one-on-one fight (or chat) that requires very high skills in certain areas.
Perhaps the best description of Planescape: Torment I've been able to find is Diane Carr's, quoted on Wikipedia, from a March 2000 article for Game Studies. She called the universe “a freak show, a long story, a zoo, and a cabinet of talkative curiosities”. If that's not enough to tickle your imagine, there's something wrong in that there cranium of yours.
If you are interested, you can get it on Good Old Games here. They've also got a very handy guide to modding the game in order to make it slightly more compatible and playable on modern systems, with a widescreen hi-res mod and a UI fix the most useful items. Read their guide here. Do read the whole of the list though, as there's even some extra quests to be added in that were left out of the original game for time reasons, but were then completed by the community.