Original Setting. Admittedly, Planescape's setting is licensed, but still, it's one of the potentially strangest fantasy settings ever, depending on what you do with it. Nat, if you read this, imagine China Mieville, only more so. Also less kinky sex. And Fallout's Mad Max/Leave it to Beaver Crossover Special isn't completely original either, but they're both distinctive and there's no other games with really similar-feeling settings, which is more than can be said about most other RPGs. Exceptions include Deus Ex, Burning Crusade (although not the rest of WoW), and, okay, I guess FFX.
Combat Doesn't Suck. Well, Planescape has the same issues that all the Baldur's Gate engine games do. But that's an acceptable combat engine, and by "acceptable," I mean "characters can move around and combat doesn't arbitrarily take place in some magical other dimension." Fallout, as I'm sure I'll mention at some point if you talk to me long enough, has the best combat system of any RPG ever. At one point during an extended firefight with some thugs, one of their stray bullets flew into the street and hit an orphan in the eye. I challenge Final Fantasy to be more compellingly awesome.
Freedom of Choice. Bioware and Bethsoft get a lot of accolades for allowing freedom of choice in their RPGs, but, as I've mentioned before, that "freedom" is generally either allocated to two linear paths (Bioware) or completely shallow and meaningless (Bethsoft). Planescape and Fallout walk the line between those two extremes really well, giving you the freedom to pursue a lot of different options, but still making those options meaningful in the long run. That won't really make a lot of sense until you've played KOTOR, Fallout, Planescape, and Morrowind all, but if you do, then you'll know what I'm talking about.
Planescape has the Best Characters Ever. Possibly in all media formats even. I will run through the list of characters:
The Nameless One: The main character. He wakes up on a mortuary slab with no memory of who he is, and discovers pretty quickly that he can't die. One of his inventory slots is "Eye," so if you happen to find a better eye during your adventure, you can upgrade. ^_^
Morte: A wisecracking floating skull who apparently was your buddy back when you had your memory. Turns out you're the reason why he's just a skull, which leads to a recurring theme in all the characters you meet: you've screwed them over somehow. Because you used to be a total bastard. His main weapon is his teeth, and yes, you will upgrade those. Fairly often.
Annah: A half-demon thief. She sounds boring by comparison to the other characters, but she's actually well-written. And hot.
Dak'kon: A serious Githzerai with a sword that changes form depending on the mind of the bearer. Again, he sounds boring but he's well-written.
Ignus: Ignus is a wizard wants to set everything on fire. As punishment for arson, his body was made into a conduit to the Elemental Plane of Fire. So now he's on fire all the time.
Nordom: A robotic Modron who was exposed to chaos and thus tainted with individuality. He dual-wields crossbows.
Fall-From-Grace: A puritan succubus. She runs the Brothel for Slaking Intellectual Lusts. She's pretty awesome.
Vhailor: A suit of armor entirely devoted to justice and order. The best tank in any game ever.
Unlike other RPGs, these characters actually feel different from each other, with fairly different mechanics, inventory, and leveling systems for each of them. Compare to Final Fantasy, where all the characters are practically generic and identical apart from a few stats.
Also Planescape is the only thing that I will not spoil the ending to. It's a great ending. Mitch, you should play it, or give it (and a PC) to Young so she could play it.
I will totally spoil the ending to Fallout 2 though: the evil mastermind is the President of the United States. And you get to set him on fire, if you like.