For me Light has a very clear plot going on Azeroth and that's Calia Menethil. Light directly intervened via S'ara (sp?) and raised her into a unique form at about the time when Blizzard was coming up with the new cosmology. She is very likely the catspaw for any Light-based schemes on Azeroth. But given that we know there is an enemy of all, a Seventh Force, it makes sense for one of the Six to betray the others and having that one be Light works just fine.
As for Life I think we will get to that plot more with the Botani who escaped on Azeroth than with Elune and the Night Elves.