According to the fact that Project Eternity is the closest thing to a sequel to Planescape: Torment we'll ever see, what will you people miss the most from the ol'game? Maybe some of it is to return (slang ? complex architecture? amnesic he... no, wait), some will probably not.
I liked the D&D Planescape's settings. Portals that could be opened anywhere at anytime, mostly accidentaly: the world felt mysterious and insecure. I also liked gods being tangible and having bad temper.
Above all, I liked the alignment system. I know, I know, this is often despised, but in Planescape, it is so absurd (the whole universe is based on alignements) it is brilliant and offers the best dialogue trick I have ever seen in a videogame: the ability to lie, in a relevant way. You know, dialogue options where you could choose "1. [truth] I like you" or "2. [lie] I like you" etc. Dialogues shaped your alignment, and in Planescape, your alignement mattered (where in Forgotten Realms/Baldur's Gate it does not do anything).
I will miss that for I see no way to redo it without copying. What about you?