One of my pet peeves is watching schoolchildren and even young adults, wearing backpacks that are hanging very low on their backs. Anyone who has done any serious backpacking (hiking, camping, Army/Marines) knows that you pull those two front straps as tight as is conveniently possible. That way, the weight is distributed more evenly across your shoulders. When it hangs low, the weight is supported by your lower back. Low-slung packs can create back pain in the short-run, and more serious problems for the long-term.
The linked article by Wendy Bumgartner, from About.COM, also has other useful links.
The proper way to wear a pack is in the first picture. Two and three shows the improper ways to do so.