The dialogue of Planescape is the reason it is my favorite game. I love how much we learn about an NPC, and how much more they come alive, just from the descriptive prose. A couple of examples of what I'd love to see more of:
Angyar: This man looks...haunted. His eyes are half-lidded, as if he has had trouble sleeping, and his hair is long and unkempt. His beard is flecked with bits of dead skin and old bits of food. He doesn't seem to notice you as you approach.
And also this, her actions outside of quotes adding so much in my opinion.
Marta: "Hah!" Marta puts her hands on her hips and puffs up indignantly. "Likes you *cared* for yer teeth!"
Brilliant!