As a frequent flying commuter it's a subject close to my heart as well...(especially AirborneSoon's point 6 - i.e. folks don't give a stuff) - Travelling on a LoCo recently I saw Cabin Crew pretty much order some poor s*d to take his legit sized baggage out of the overhead locker and into his foot space so that somebody else could jam their most definitely oversized backpack into the overhead.
Even when traveling with "my" airline, which still assigns seats, you get the LoCo scrum at the gate as people shove be first on in order to stow their shopping. I reckon the only real answer is each seat has it's own assigned slot in the overhead locker and if your handbaggage won't go there, or under your seat, it goes to the hold.....but I know it's not going to happen.