You've probably read about how much story and text Planescape throws at you.  Some might consider this a bad thing, but I can vouch that all of verbiage they give you is GOOD.  I knew this was a different, better kind of game when I started out in a mausoleum filled with zombies.  Not just repetitive undead attackers, though -- each zombie had its own description and characteristics, and for the first time in a game I got the feeling that these undead were actual former people -- very creepy.  Farther on you are exposed to the troubled pasts of the main character and his cohorts, but not all at once; your companions react to each other and the things that happen to you.  Talk to them periodically and different options open up.  Not to mention all of the existential issues that are explored with the main character, who is immortal but can't remember his past incarnations.  Some people have noted that maybe P:T presents you with too much storyline, and doesn't let you role-play enough, but there is still ample opportunity to affect your character's standing both with your stats (wisdom is important even though your character can only be a fighter, mage, and thief) and with your actions within the game.