One thing that strikes me when I look at the Saturn launch vehicle designs is the enormous amount of helium they used for purging and pressurization, particularly of fuel tanks (gaseous oxygen seems to have been used for pressurizing the LOX tanks).
Besides its light weight, helium does have the unique property of remaining gaseous even at liquid hydrogen temperatures, allowing it to be stored in spheres inside the LH2 tank in the S-IVB, for example.
But anybody who has tried to get helium lately for saturation diving, welding, filling balloons, etc, knows just how bloody scarce and expensive the stuff has become. This is not actually a big surprise since it's a limited, non-renewable resource (on human timescales, anyway) and we've been using the stuff pretty indiscriminately.
I suppose helium is still acceptable in smaller pressure-fed hypergolic engines on spacecraft, but using huge amounts in launch vehicles seems like a real sustainability problem. Does anybody know if modern space launch vehicles still use it to the degree that the Saturns did?