Lying is important. In Planescape, it was common that two or three of the dialogue options one had was marked with [Lie]. You could lie about many things, not simply quest things like "Did Leandro steal my things?" but lying about personal beliefs, intentions, etc.
I've seen other RPGs incorporate lying, but only P:T would have the same identical lines available, one marked with [Lie] and the other without.
I also understand it would be hard work to incorporate it into the game world, but the ability to spread false rumours, create arbitrary conflicts, and in general get factions, orga