When I use a backpack, I'm always quite careful not to bump it into people. But by the same token, I wish people behind me in the checkin line wouldn't walk so close to my pack that they bump into it which, of course, means bumping into me as well. Also, it's not unusual for people in line pulling a roller bag to suddenly back up enough to bang the bag into the foot and/or shin of the person behind them.

Bottom line: everybody should kindly realize that all their luggage occupies space. Thank you.