I'm a little late to this conversation, but thought I'd toss my bit in as well.
I would say there are dual drivers in play in the series.
First, there is Londo. The decions he made for essentially two reasons (the Centauri returning to emminence, and raising his own profile, and House Mollari) caused an emormous amount of the plot to move forward.
Second, there is Delenn. She is the "light" for Londo's "dark." I'm not implying that Londo was evil, but he started down the path with Morden and had little concern for the effects of Morden and the Shadows deeds in his name for a long time. Delenn drove the majority of the light action, developing Sheridan and G'Kar as allies to defeat the Shadows (and in an way, the Volrlons) while being Sheridan's booster/protector/lover and letting him be the big "hero."
Yes, such an ensemble that all were necessary in big and little ways. It is the great strength of this story, that we can't imagine it without the majority of the cast/characters.
P.S. Welcome, My Name Is Lyta.