But the fact that Lelouch is the head of an organization, in contrast with Light, who workes alone, makes Lelouch a much more interesting character, facing more diverse moral problems.
Light's character was quite simple: some smart guy who can kill anyone as long as he has a name and a face. with the police after him. period.
Indeed, his plans rocked, he managed to control people/ shinigami based on their feelings and personality. But he himself does not have human "traits", or at least not that apparent. The only moment I felt he was actually human resulting in a mistake from his part was when he wasn't able to kill his sister. But that was all.
Lelouch, on the other hand, is a more complex character. His love for his sister, his friendship with suzaku and all those diverse relationships with different characters in the serie make him so much different than Light.
I think that's the whole charm of the serie. Plus, the geass IS the plot. The whole story revolves around how people are changed because of it. AND how its own host changes because of it.but the bad thing about geass is people doing things completely out of character (lelouch included) and that's a cheap way of getiing on with the plot imo.