There usually are but with time constraints and whatnot, content is always cut. I can't recall them ever mentioning any specifics regarding what was cut from Halo 1's campaign aside from weapons, but I do remember them saying that Halo 2 allowed them to flush out a ideas/plans leftover from the first game. I know this is a pretty vague answer but I think it's just about the best anyone who didn't work on Halo:CE can give you.