In KOTR, most of the Dark Side choices involved picking dialoge choices that, to put it bluntly, made you seem more like a jerk and not really a power seeking Dark Sider. I think it would be really awsome if the major plot, and some side-quests, allowed you to chose responses that kinda of blew someone off. It just seems to me that a majority of Dark Side reponses involve you garnaring for mone credits or an extra reward. I think a true Sith could care less, s/he would be more concerned with tracking down that star forge and gaining power. More decit and guile for Dark Siders, I'm not a petty criminal I'm a Dark Side apprentice.
Examples for main plot dialogs could involve trciking and manipulating NPCs, even your own party members, to create confusion and distrust. Maybe create a way to travel the Dark Side path by completing quests seemingly on the Light Side, only because you wanted to advance as quickly and secretly as possible.
For example, a possible side-quest; imagine some stranger walks up to you and asks for help to bring water for their son's dying flower plot. Your character would keep trying to ignore the person, but they would keep bugging you, so eventually you just force choke them until they leave you alone. (On a side-note, it would also be cool if force choke could be used as a persuasian method much like affect mind...evil!)
There were a couple of instances in KOTR that you could act truely evil before the unknown planet, but not many. I'm just rambling, but let's get some thoughts on this.