No backpacks after college or grad school.
Yes, I know there are arguably stylish ones out there, but I'm talking about the big canvas Jansport variety that students lug their textbooks around in. You don't need to lug your life around in these once you've put your textbooks away, and it's really not good for your back anyway. This is especially annoying on crowded public transportation, where people take up valuable aisle space wearing these huge, overstuffed backpacks on their backs. Men seem to be especially guilty of this one, and it makes me cringe to see them in nice business suits with big backpacks slung over their shoulders.
Of course, I can also make an exception for hikers and people who are actually "backpacking" when they travel.